diff --git a/backend/app/api_notifications.py b/backend/app/api_notifications.py
index c431b43..0e3e255 100644
--- a/backend/app/api_notifications.py
+++ b/backend/app/api_notifications.py
@@ -443,6 +443,40 @@ def make_router(config: Config) -> APIRouter:
 
     # ----- Email: one-click unsubscribe + bounce webhook -----
 
+    # v0.18.0: the category → column map. The `all` synthetic
+    # category lands the bundle's one-click on the global opt-out
+    # flag (per `email._send_bundle` in v0.18.0 Slice 2 — a bundle
+    # spans multiple categories, so a per-category flip wouldn't
+    # honor the user's intent).
+    _CATEGORY_COLUMN: dict[str, str] = {
+        "personal-direct": "email_personal_direct",
+        "structural": "email_watched_structural",
+        "admin-actionable": "email_admin_actionable",
+        "all": "email_opt_out_all",
+    }
+
+    def _apply_unsubscribe(user_id: int, category: str) -> bool:
+        """Flip the matching column. Returns True on success, False
+        if the category is unknown. Idempotent — running twice on
+        the same (user, category) is harmless (it sets the column
+        to its current value)."""
+        column = _CATEGORY_COLUMN.get(category)
+        if column is None:
+            return False
+        # `all` sets the flag to 1 (opt out); per-category sets to 0
+        # (turn that category off). The column semantic is "1 means
+        # don't send"; the per-category booleans are "1 means do
+        # send". Different polarities, hence the case split.
+        if category == "all":
+            db.conn().execute(
+                f"UPDATE users SET {column} = 1 WHERE id = ?", (user_id,)
+            )
+        else:
+            db.conn().execute(
+                f"UPDATE users SET {column} = 0 WHERE id = ?", (user_id,)
+            )
+        return True
+
     @router.get("/api/email/unsubscribe")
     async def email_unsubscribe(t: str = Query(..., description="Signed token from the email footer")) -> HTMLResponse:
         try:
@@ -453,20 +487,51 @@ def make_router(config: Config) -> APIRouter:
                 "<p>Open the app to manage your notification preferences directly.</p>",
                 status_code=400,
             )
-        column = {
-            "personal-direct": "email_personal_direct",
-            "structural": "email_watched_structural",
-            "admin-actionable": "email_admin_actionable",
-        }.get(category)
-        if column is None:
+        if not _apply_unsubscribe(user_id, category):
             return HTMLResponse(
                 f"<h1>Unknown category</h1><p>{category}</p>", status_code=400
             )
-        db.conn().execute(f"UPDATE users SET {column} = 0 WHERE id = ?", (user_id,))
-        return HTMLResponse(
-            f"<h1>Unsubscribed</h1><p>You will no longer receive {category} emails. "
-            f"You can re-enable them in your notification preferences.</p>"
-        )
+        if category == "all":
+            body = (
+                "<h1>Unsubscribed</h1><p>You will no longer receive any email "
+                "from this app. You can re-enable individual categories from "
+                "your notification preferences after signing in.</p>"
+            )
+        else:
+            body = (
+                f"<h1>Unsubscribed</h1><p>You will no longer receive {category} emails. "
+                f"You can re-enable them in your notification preferences.</p>"
+            )
+        return HTMLResponse(body)
+
+    @router.post("/api/email/unsubscribe")
+    async def email_unsubscribe_post(
+        request: Request,
+        t: str = Query(..., description="Signed token from the List-Unsubscribe header"),
+    ) -> dict[str, Any]:
+        """v0.18.0: RFC 8058 one-click endpoint.
+
+        Gmail and Yahoo POST `List-Unsubscribe=One-Click` (as a
+        form-encoded body) to the URL in the `List-Unsubscribe`
+        header when the user clicks their MUA's "Unsubscribe"
+        button. The endpoint MUST accept POST (per the
+        `List-Unsubscribe-Post` header we advertise) and MUST be
+        idempotent.
+
+        The body content is checked loosely — RFC 8058 says it
+        SHOULD be exactly `List-Unsubscribe=One-Click`, but some
+        intermediaries strip / re-encode the body, so the
+        framework accepts any POST to the URL once the token
+        verifies. The bar is that the token signature carries the
+        authority; the body is hint-only.
+        """
+        try:
+            user_id, category = email_mod.verify_unsubscribe_token(t)
+        except BadSignature:
+            raise HTTPException(400, "Invalid or expired token")
+        if not _apply_unsubscribe(user_id, category):
+            raise HTTPException(400, f"Unknown category: {category}")
+        return {"ok": True, "category": category}
 
     @router.post("/api/webhooks/email-bounce")
     async def email_bounce(body: BounceBody, request: Request) -> dict[str, Any]:
diff --git a/backend/app/digest.py b/backend/app/digest.py
index 3af63b0..1e91de9 100644
--- a/backend/app/digest.py
+++ b/backend/app/digest.py
@@ -180,7 +180,18 @@ def assemble_for_user(
 
     subject = _subject(eligible, cadence)
     body = _body(eligible, cadence, cfg)
-    sent = email_mod._deliver(cfg, email, subject, body)
+    # v0.18.0: the digest is the bulk-adjacent surface par excellence
+    # (it can carry weeks of accumulated activity), so it gets the
+    # full one-click unsubscribe to the global opt-out. Per-category
+    # opt-outs are managed from the preferences page; this footer is
+    # the "stop sending me anything" escape hatch Gmail and Yahoo
+    # expect for senders at this tier.
+    unsubscribe_url = email_mod.make_unsubscribe_url(user_id, "all")
+    sent = email_mod._deliver(
+        cfg, email, subject, body,
+        unsubscribe_mailto=cfg.unsubscribe_mailto,
+        unsubscribe_url=unsubscribe_url,
+    )
     if not sent:
         return False
     ids = [r["id"] for r, _ in eligible]
diff --git a/backend/app/email.py b/backend/app/email.py
index fdd0e93..5e150b2 100644
--- a/backend/app/email.py
+++ b/backend/app/email.py
@@ -24,7 +24,6 @@ import os
 import smtplib
 from dataclasses import dataclass
 from datetime import datetime, time, timezone
-from email.message import EmailMessage
 from email.utils import formataddr
 from itertools import groupby
 from typing import Any
@@ -33,6 +32,7 @@ from urllib.parse import urlencode
 from itsdangerous import BadSignature, URLSafeSerializer
 
 from . import db
+from .email_envelope import build_envelope
 
 log = logging.getLogger(__name__)
 
@@ -69,6 +69,7 @@ class EmailConfig:
     app_url: str
     bundle_threshold: int
     enabled: bool
+    unsubscribe_mailto: str
 
     @classmethod
     def from_env(cls) -> "EmailConfig":
@@ -84,6 +85,16 @@ class EmailConfig:
             app_url=os.environ.get("APP_URL", "http://localhost:8000").rstrip("/"),
             bundle_threshold=int(os.environ.get("EMAIL_BUNDLE_THRESHOLD", "5")),
             enabled=os.environ.get("EMAIL_ENABLED", "1") not in ("0", "false", "False"),
+            # v0.18.0: the `List-Unsubscribe: <mailto:…>` target on
+            # invite + notification mail. Defaults to the From
+            # address when unset; a deployment can route opt-out
+            # mail to a separate mailbox (e.g., a humans-monitored
+            # account distinct from the no-reply notifications
+            # sender) by setting this explicitly.
+            unsubscribe_mailto=os.environ.get(
+                "EMAIL_UNSUBSCRIBE_MAILTO",
+                os.environ.get("EMAIL_FROM", "notifications@wiggleverse.local"),
+            ).strip(),
         )
 
 
@@ -98,6 +109,14 @@ def _signer() -> URLSafeSerializer:
 
 
 def make_unsubscribe_url(user_id: int, category: str) -> str:
+    """Build the §15.4 per-category one-click URL.
+
+    `category` is one of `personal-direct`, `structural`,
+    `admin-actionable` (the three per-category flags) or `all`
+    (v0.18.0: the bundle path, which sets `email_opt_out_all = 1`
+    because a bundle covers multiple categories and a per-category
+    opt-out wouldn't honor the user's intent).
+    """
     cfg = EmailConfig.from_env()
     token = _signer().dumps({"u": user_id, "c": category})
     qs = urlencode({"t": token})
@@ -250,7 +269,19 @@ def _send_one(user: Any, notif_id: int, payload: dict, category: str) -> None:
         return
     subject = _subject(payload)
     body = _body(payload, user["id"], category, cfg)
-    sent = _deliver(cfg, user["email"], subject, body)
+    # v0.18.0: notification mail is bulk-adjacent (a watcher can
+    # accumulate dozens of structural events on a busy RFC), so it
+    # carries the full one-click unsubscribe — Gmail and Yahoo
+    # require this for senders at OHM's volume tier per RFC 8058.
+    unsubscribe_url = make_unsubscribe_url(user["id"], category)
+    sent = _deliver(
+        cfg,
+        user["email"],
+        subject,
+        body,
+        unsubscribe_mailto=cfg.unsubscribe_mailto,
+        unsubscribe_url=unsubscribe_url,
+    )
     if not sent:
         return
     db.conn().execute(
@@ -305,23 +336,45 @@ def _deep_link(payload: dict, cfg: EmailConfig) -> str:
     return cfg.app_url
 
 
-def _deliver(cfg: EmailConfig, to_address: str, subject: str, body: str) -> bool:
+def _deliver(
+    cfg: EmailConfig,
+    to_address: str,
+    subject: str,
+    body: str,
+    *,
+    unsubscribe_mailto: str | None = None,
+    unsubscribe_url: str | None = None,
+) -> bool:
+    """Build the envelope via the shared `build_envelope` helper and
+    hand it to SMTP.
+
+    The `_SENT` buffer carries the helper's `EmailMessage` under
+    `message` plus the legacy `to`/`from`/`subject`/`body` keys for
+    backward-compatibility with tests that read those directly.
+    Newer tests can assert on the header surface by inspecting
+    `envelope["message"]`.
+    """
+    msg = build_envelope(
+        to_address=to_address,
+        from_address=cfg.from_address,
+        from_name=cfg.from_name,
+        subject=subject,
+        body_plain=body,
+        unsubscribe_mailto=unsubscribe_mailto,
+        unsubscribe_url=unsubscribe_url,
+    )
     envelope = {
         "to": to_address,
         "from": formataddr((cfg.from_name, cfg.from_address)),
         "subject": subject,
         "body": body,
+        "message": msg,
     }
     _SENT.append(envelope)
     if not cfg.smtp_host:
         log.info("email (stdout fallback): to=%s subject=%s", to_address, subject)
         return True
     try:
-        msg = EmailMessage()
-        msg["From"] = envelope["from"]
-        msg["To"] = to_address
-        msg["Subject"] = subject
-        msg.set_content(body)
         smtp = smtplib.SMTP(cfg.smtp_host, cfg.smtp_port, timeout=30)
         try:
             if cfg.smtp_starttls:
@@ -440,13 +493,26 @@ def _send_bundle(cfg: EmailConfig, user: Any, emailable: list) -> int:
         for r, _cat, extras in group_rows:
             summary = _summary_for(r["event_kind"], r["actor_display"], r["rfc_title"], extras)
             sections.append(f"  · {summary}")
+    # v0.18.0: the bundle covers multiple categories, so a
+    # per-category opt-out can't honor the user's intent. The
+    # `all` category lands at the §15.4 endpoint and sets
+    # `email_opt_out_all = 1`.
+    unsubscribe_url = make_unsubscribe_url(user["id"], "all")
     body = (
         "Activity on RFCs you watch, accumulated during your quiet hours:\n"
         + "\n".join(sections)
         + f"\n\nOpen your inbox: {cfg.app_url}/inbox\n"
         + f"Manage all preferences: {cfg.app_url}/settings/notifications\n"
+        + f"Unsubscribe from all email: {unsubscribe_url}\n"
+    )
+    sent = _deliver(
+        cfg,
+        user["email"],
+        subject,
+        body,
+        unsubscribe_mailto=cfg.unsubscribe_mailto,
+        unsubscribe_url=unsubscribe_url,
     )
-    sent = _deliver(cfg, user["email"], subject, body)
     if not sent:
         return 0
     ids = [r["id"] for r, _, _ in emailable]
diff --git a/backend/app/email_invite.py b/backend/app/email_invite.py
index 9b9d96f..31a13dd 100644
--- a/backend/app/email_invite.py
+++ b/backend/app/email_invite.py
@@ -31,10 +31,10 @@ from __future__ import annotations
 
 import logging
 import smtplib
-from email.message import EmailMessage
 from email.utils import formataddr
 
 from .email import EmailConfig, _SENT
+from .email_envelope import build_envelope
 
 log = logging.getLogger(__name__)
 
@@ -59,12 +59,29 @@ def send_invite_email(
     cfg = EmailConfig.from_env()
     subject = _subject(inviter_display, cfg)
     body = _body(claim_url, inviter_display, inviter_email, custom_message, cfg)
+    # v0.18.0: invite mail carries a `List-Unsubscribe: <mailto:…>`
+    # only (no signed URL) — the invitee isn't a user yet, so there
+    # is no per-user opt-out row to flip. The operator handles
+    # ad-hoc opt-outs from the mailto: target. Per the proposal's
+    # "Tradeoff discussion": the invite was unsolicited from the
+    # recipient's perspective, so the courtesy header is right;
+    # but it can't be a one-click URL because the row doesn't
+    # exist yet.
+    msg = build_envelope(
+        to_address=to_address,
+        from_address=cfg.from_address,
+        from_name=cfg.from_name,
+        subject=subject,
+        body_plain=body,
+        unsubscribe_mailto=cfg.unsubscribe_mailto,
+    )
     envelope = {
         "to": to_address,
         "from": formataddr((cfg.from_name, cfg.from_address)),
         "subject": subject,
         "body": body,
         "kind": "invite",
+        "message": msg,
     }
     _SENT.append(envelope)
 
@@ -79,11 +96,6 @@ def send_invite_email(
         return True
 
     try:
-        msg = EmailMessage()
-        msg["From"] = envelope["from"]
-        msg["To"] = to_address
-        msg["Subject"] = subject
-        msg.set_content(body)
         smtp = smtplib.SMTP(cfg.smtp_host, cfg.smtp_port, timeout=30)
         try:
             if cfg.smtp_starttls:
diff --git a/backend/app/email_otc.py b/backend/app/email_otc.py
index b2f59d9..2d8b461 100644
--- a/backend/app/email_otc.py
+++ b/backend/app/email_otc.py
@@ -23,10 +23,10 @@ from __future__ import annotations
 
 import logging
 import smtplib
-from email.message import EmailMessage
 from email.utils import formataddr
 
 from .email import EmailConfig, _SENT
+from .email_envelope import build_envelope
 
 log = logging.getLogger(__name__)
 
@@ -44,12 +44,25 @@ def send_otc_email(to_address: str, code: str) -> bool:
     cfg = EmailConfig.from_env()
     subject = f"Your sign-in code for {cfg.from_name}"
     body = _body(code, cfg)
+    # v0.18.0: OTC mail carries NO List-Unsubscribe — the recipient
+    # explicitly requested the code; advertising an unsubscribe
+    # header would imply OHM has them on a list, which it doesn't.
+    # See the proposal's "Tradeoff discussion" for the binding
+    # rationale.
+    msg = build_envelope(
+        to_address=to_address,
+        from_address=cfg.from_address,
+        from_name=cfg.from_name,
+        subject=subject,
+        body_plain=body,
+    )
     envelope = {
         "to": to_address,
         "from": formataddr((cfg.from_name, cfg.from_address)),
         "subject": subject,
         "body": body,
         "kind": "otc",
+        "message": msg,
     }
     _SENT.append(envelope)
 
@@ -64,11 +77,6 @@ def send_otc_email(to_address: str, code: str) -> bool:
         return True
 
     try:
-        msg = EmailMessage()
-        msg["From"] = envelope["from"]
-        msg["To"] = to_address
-        msg["Subject"] = subject
-        msg.set_content(body)
         smtp = smtplib.SMTP(cfg.smtp_host, cfg.smtp_port, timeout=30)
         try:
             if cfg.smtp_starttls:
diff --git a/backend/tests/test_admin_create_user_invite_vertical.py b/backend/tests/test_admin_create_user_invite_vertical.py
index a160f2e..343e5fd 100644
--- a/backend/tests/test_admin_create_user_invite_vertical.py
+++ b/backend/tests/test_admin_create_user_invite_vertical.py
@@ -647,3 +647,82 @@ def test_pending_invites_listing_admin_only(app_with_fake_gitea):
         )
         r = client.get("/api/admin/users/invites")
         assert r.status_code == 403
+
+
+# ---------------------------------------------------------------------------
+# v0.18.0: invite-envelope header shape — Slice 2
+#
+# Invite mail goes through `build_envelope` and MUST land Date,
+# Message-ID, Auto-Submitted, AND a `List-Unsubscribe: <mailto:…>`
+# (no URL — the invitee isn't a user yet, so no per-user opt-out
+# row exists). The mailto: target is the operator's `EMAIL_FROM`
+# by default; the operator can override via `EMAIL_UNSUBSCRIBE_MAILTO`.
+# ---------------------------------------------------------------------------
+
+
+def _provision_admin_and_send_invite(client, app_with_fake_gitea_fixture, *, to: str = "headers@ex.co"):
+    provision_user_row(user_id=400, login="adminH", role="admin")
+    sign_in_as(
+        client, user_id=400, gitea_login="adminH",
+        display_name="Admin H", role="admin",
+        email="adminh@test",
+    )
+    _reset_outbound()
+    r = client.post(
+        "/api/admin/users",
+        json={
+            "email": to,
+            "first_name": "Header",
+            "last_name": "Test",
+            "role": "contributor",
+            "custom_message": "",
+        },
+    )
+    assert r.status_code == 200, r.text
+    return _outbound_invite_envelopes(to)[-1]
+
+
+def test_invite_envelope_sets_date_messageid_autosubmitted(app_with_fake_gitea):
+    from fastapi.testclient import TestClient
+    from email.utils import parsedate_to_datetime
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        env = _provision_admin_and_send_invite(client, (app, _fake))
+        msg = env["message"]
+        assert parsedate_to_datetime(msg["Date"]) is not None
+        assert msg["Message-ID"].startswith("<") and msg["Message-ID"].endswith(">")
+        assert msg["Auto-Submitted"] == "auto-generated"
+
+
+def test_invite_envelope_has_mailto_list_unsubscribe_only(app_with_fake_gitea):
+    """The invitee isn't a user yet — no per-user opt-out URL is
+    available. The `List-Unsubscribe` MUST be a mailto: form, and
+    the `List-Unsubscribe-Post` header MUST be absent (the
+    one-click semantic requires a URL the MUA can POST to)."""
+    from fastapi.testclient import TestClient
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        env = _provision_admin_and_send_invite(client, (app, _fake))
+        msg = env["message"]
+        lu = msg["List-Unsubscribe"]
+        assert lu is not None and lu.startswith("<mailto:")
+        # No URL part — invite is mailto-only.
+        assert "https://" not in lu and "http://" not in lu
+        assert msg["List-Unsubscribe-Post"] is None
+
+
+def test_invite_envelope_respects_email_unsubscribe_mailto_override(app_with_fake_gitea, monkeypatch):
+    """When `EMAIL_UNSUBSCRIBE_MAILTO` is set, the mailto: target on
+    `List-Unsubscribe` honors it (lets a deployment route opt-outs
+    to a humans-monitored mailbox distinct from the no-reply
+    sender)."""
+    from fastapi.testclient import TestClient
+
+    monkeypatch.setenv("EMAIL_UNSUBSCRIBE_MAILTO", "ohm@wiggleverse.org?subject=remove")
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        env = _provision_admin_and_send_invite(client, (app, _fake))
+        msg = env["message"]
+        assert "ohm@wiggleverse.org?subject=remove" in msg["List-Unsubscribe"]
diff --git a/backend/tests/test_notifications_vertical.py b/backend/tests/test_notifications_vertical.py
index 926d846..acc0543 100644
--- a/backend/tests/test_notifications_vertical.py
+++ b/backend/tests/test_notifications_vertical.py
@@ -612,3 +612,127 @@ def test_explicit_watch_set_overrides_auto(app_with_fake_gitea):
         # the user put them.
         assert row["set_by"] == "explicit"
         assert row["state"] == "following"
+
+
+# ---------------------------------------------------------------------------
+# v0.18.0 — envelope headers + RFC 8058 one-click POST endpoint
+#
+# Watcher notifications are bulk-adjacent (a busy RFC can produce
+# dozens of structural events); per the proposal, they MUST carry
+# `Date`, `Message-ID`, `Auto-Submitted`, full `List-Unsubscribe`
+# (mailto + signed URL), AND `List-Unsubscribe-Post:
+# List-Unsubscribe=One-Click` per RFC 8058. Gmail and Yahoo
+# enforce this for senders at OHM's volume tier.
+# ---------------------------------------------------------------------------
+
+
+def test_notification_envelope_carries_full_one_click_headers(app_with_fake_gitea):
+    """A `proposal_merged` event lands a watcher notification email
+    with the full one-click unsubscribe shape."""
+    from fastapi.testclient import TestClient
+    from email.utils import parsedate_to_datetime
+    from app import db, email as email_mod
+
+    app, fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        provision_user_row(user_id=2, login="alice", role="contributor")
+        provision_user_row(user_id=1, login="ben", role="owner")
+
+        sign_in_as(client, user_id=2, gitea_login="alice", display_name="Alice", role="contributor", email="alice@test")
+        r = client.post("/api/rfcs/propose", json={"title": "OHM", "slug": "ohm", "pitch": PITCH, "tags": []})
+        assert r.status_code == 200
+        email_mod.reset_sent_envelopes()
+        sign_in_as(client, user_id=1, gitea_login="ben", display_name="Ben", role="owner", email="ben@test")
+        merge_r = client.post(f"/api/proposals/{r.json()['pr_number']}/merge")
+        assert merge_r.status_code == 200, merge_r.text
+
+        envelopes = [e for e in email_mod.sent_envelopes() if e["to"] == "alice@test"]
+        assert envelopes, "watcher notification did not fire"
+        msg = envelopes[-1]["message"]
+        # Always-present headers from the helper.
+        assert parsedate_to_datetime(msg["Date"]) is not None
+        assert msg["Message-ID"].startswith("<") and msg["Message-ID"].endswith(">")
+        assert msg["Auto-Submitted"] == "auto-generated"
+        # Full one-click unsubscribe.
+        lu = msg["List-Unsubscribe"]
+        assert lu is not None
+        assert "<mailto:" in lu
+        # URL part carries the signed token per make_unsubscribe_url.
+        assert "/api/email/unsubscribe?t=" in lu
+        assert msg["List-Unsubscribe-Post"] == "List-Unsubscribe=One-Click"
+
+
+def test_email_unsubscribe_post_one_click_flips_category_off(app_with_fake_gitea):
+    """RFC 8058: Gmail/Yahoo POST `List-Unsubscribe=One-Click` to the
+    URL in the List-Unsubscribe header. The endpoint MUST accept POST
+    + the same token shape as the GET handler + return 200 + flip the
+    flag."""
+    from fastapi.testclient import TestClient
+    from app import db, email as email_mod
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        provision_user_row(user_id=2, login="alice", role="contributor")
+        token = email_mod.make_unsubscribe_url(2, "personal-direct").split("t=", 1)[1]
+
+        r = client.post(
+            f"/api/email/unsubscribe?t={token}",
+            data={"List-Unsubscribe": "One-Click"},
+        )
+        assert r.status_code == 200, r.text
+        body = r.json()
+        assert body["ok"] is True
+        assert body["category"] == "personal-direct"
+
+        row = db.conn().execute(
+            "SELECT email_personal_direct FROM users WHERE id = 2"
+        ).fetchone()
+        assert row["email_personal_direct"] == 0
+
+
+def test_email_unsubscribe_post_all_sets_global_opt_out(app_with_fake_gitea):
+    """The v0.18.0 `all` synthetic category (used by the bundle +
+    digest paths) MUST set `email_opt_out_all = 1`."""
+    from fastapi.testclient import TestClient
+    from app import db, email as email_mod
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        provision_user_row(user_id=2, login="alice", role="contributor")
+        token = email_mod.make_unsubscribe_url(2, "all").split("t=", 1)[1]
+        r = client.post(f"/api/email/unsubscribe?t={token}")
+        assert r.status_code == 200
+        assert r.json() == {"ok": True, "category": "all"}
+        row = db.conn().execute(
+            "SELECT email_opt_out_all FROM users WHERE id = 2"
+        ).fetchone()
+        assert row["email_opt_out_all"] == 1
+
+
+def test_email_unsubscribe_get_all_sets_global_opt_out(app_with_fake_gitea):
+    """GET handler also accepts the `all` category and lands the
+    global opt-out (so an MUA that doesn't honor RFC 8058 POST and
+    just opens the URL in a browser still works)."""
+    from fastapi.testclient import TestClient
+    from app import db, email as email_mod
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        provision_user_row(user_id=2, login="alice", role="contributor")
+        token = email_mod.make_unsubscribe_url(2, "all").split("t=", 1)[1]
+        r = client.get(f"/api/email/unsubscribe?t={token}")
+        assert r.status_code == 200
+        assert "Unsubscribed" in r.text
+        row = db.conn().execute(
+            "SELECT email_opt_out_all FROM users WHERE id = 2"
+        ).fetchone()
+        assert row["email_opt_out_all"] == 1
+
+
+def test_email_unsubscribe_post_refuses_invalid_token(app_with_fake_gitea):
+    from fastapi.testclient import TestClient
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        r = client.post("/api/email/unsubscribe?t=not-a-valid-token")
+        assert r.status_code == 400
diff --git a/backend/tests/test_otc_vertical.py b/backend/tests/test_otc_vertical.py
index ab77588..1d0cb7d 100644
--- a/backend/tests/test_otc_vertical.py
+++ b/backend/tests/test_otc_vertical.py
@@ -347,3 +347,53 @@ def test_otc_re_request_invalidates_prior_unused_code(app_with_fake_gitea, monke
         # The new code still works.
         r = client.post("/auth/otc/verify", json={"email": "alice@example.com", "code": second})
         assert r.status_code == 200
+
+
+# ---------------------------------------------------------------------------
+# v0.18.0: envelope headers — Slice 2
+#
+# OTC mail goes through `build_envelope` and MUST land Date,
+# Message-ID, and Auto-Submitted but MUST NOT carry a
+# List-Unsubscribe header (the recipient explicitly requested the
+# code; advertising a list semantic would be wrong).
+# ---------------------------------------------------------------------------
+
+
+def _last_otc_envelope():
+    from app import email as email_mod
+    otc = [e for e in email_mod.sent_envelopes() if e.get("kind") == "otc"]
+    assert otc, "no OTC envelope in the buffer"
+    return otc[-1]
+
+
+def test_otc_envelope_sets_date_messageid_autosubmitted(app_with_fake_gitea):
+    from fastapi.testclient import TestClient
+    from email.utils import parsedate_to_datetime
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        _reset_outbound()
+        client.post("/auth/otc/request", json={"email": "headers@example.com"})
+        msg = _last_otc_envelope()["message"]
+        # Date is RFC 5322 parseable.
+        assert parsedate_to_datetime(msg["Date"]) is not None
+        # Message-ID is bracketed and carries the From-address @-domain.
+        mid = msg["Message-ID"]
+        assert mid.startswith("<") and mid.endswith(">")
+        # Auto-Submitted prevents auto-responder loops.
+        assert msg["Auto-Submitted"] == "auto-generated"
+
+
+def test_otc_envelope_has_no_list_unsubscribe(app_with_fake_gitea):
+    """The recipient explicitly typed their email and asked for a
+    code; the framework MUST NOT advertise a list semantic on this
+    mail. Per the v0.18.0 proposal's tradeoff discussion."""
+    from fastapi.testclient import TestClient
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        _reset_outbound()
+        client.post("/auth/otc/request", json={"email": "headers@example.com"})
+        msg = _last_otc_envelope()["message"]
+        assert msg["List-Unsubscribe"] is None
+        assert msg["List-Unsubscribe-Post"] is None