Add opt-in tenant-to-client bindings for invitation mail

User-engine side of EMAIL-WP-0008: USER_ENGINE_MAIL_CLIENT_BINDINGS maps
tenant globs to a single client_id carried on invitation mail envelopes
only. Unmatched or conflicting tenants fail as mail_binding_denied before
either lane is contacted, and the outbox now records that reason instead
of collapsing it to delivery_unavailable. Unset keeps legacy envelopes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 445231@bnt-lap001
Assistant-Session: 024a807e-0cc5-4f2c-bc8a-b83322d1395d

src/user_engine/adapters/delivery.py

@@ -19,6 +19,7 @@ class NoRedirect(HTTPRedirectHandler):
 
 
 from user_engine.domain import OutboxEvent
+from user_engine.mail_bindings import load_mail_client_bindings, mail_client_for
 
 _MAIL_EVENTS = {"family_member.invited", "family_invitation.resent"}
 
@@ -34,12 +35,14 @@ class HTTPOutboxDeliveryAdapter:
         mail_url: str | None = None,
         mail_bearer_token: str | None = None,
         timeout_seconds: float = 5.0,
+        mail_client_bindings: str | None = None,
     ) -> None:
         self.event_url = event_url
         self.mail_url = mail_url
         self.event_bearer_token = event_bearer_token
         self.mail_bearer_token = mail_bearer_token
         self.timeout_seconds = timeout_seconds
+        self.mail_client_bindings = load_mail_client_bindings(mail_client_bindings)
 
     def __call__(self, event: OutboxEvent) -> None:
         envelope = {
@@ -57,7 +60,15 @@ class HTTPOutboxDeliveryAdapter:
         if self.mail_url and event.event_type in _MAIL_EVENTS:
             if not self.mail_bearer_token:
                 raise RuntimeError("mail delivery token is required")
-            self._post(self.mail_url, envelope, self.mail_bearer_token)
+            mail_envelope = dict(envelope)
+            if self.mail_client_bindings is not None:
+                try:
+                    mail_envelope["client_id"] = mail_client_for(
+                        event.tenant, self.mail_client_bindings
+                    )
+                except ValueError:
+                    raise DeliveryError("mail_binding_denied") from None
+            self._post(self.mail_url, mail_envelope, self.mail_bearer_token)
         event_envelope = dict(envelope)
         event_data = dict(envelope["data"])
         if "primary_email" in event_data:

src/user_engine/mail_bindings.py

@@ -0,0 +1,40 @@
+"""Explicit tenant globs for the scoped invitation mail transport."""
+
+from __future__ import annotations
+
+import json
+import re
+from fnmatch import fnmatchcase
+
+IDENTIFIER = re.compile(r"[A-Za-z0-9][A-Za-z0-9._:-]{0,127}\Z")
+PATTERN = re.compile(r"[A-Za-z0-9*?][A-Za-z0-9._:*?-]{0,127}\Z")
+
+
+def load_mail_client_bindings(raw):
+    if raw is None:
+        return None
+    try:
+        rows = json.loads(raw)
+        if not isinstance(rows, list) or not 1 <= len(rows) <= 128:
+            raise ValueError
+        result = []
+        for row in rows:
+            if (not isinstance(row, list) or len(row) != 2
+                    or not isinstance(row[0], str) or not PATTERN.fullmatch(row[0])
+                    or not isinstance(row[1], str) or not IDENTIFIER.fullmatch(row[1])):
+                raise ValueError
+            result.append(tuple(row))
+        if len({row[0] for row in result}) != len(result):
+            raise ValueError
+        return tuple(result)
+    except (ValueError, TypeError):
+        raise ValueError("mail_client_bindings_invalid") from None
+
+
+def mail_client_for(tenant, bindings):
+    if not isinstance(tenant, str) or not IDENTIFIER.fullmatch(tenant):
+        raise ValueError("mail_binding_denied")
+    matches = {client for pattern, client in bindings if fnmatchcase(tenant, pattern)}
+    if len(matches) != 1:
+        raise ValueError("mail_binding_denied")
+    return matches.pop()

src/user_engine/runtime.py

@@ -65,6 +65,7 @@ def create_application() -> PortalApplication:
             else None
         ),
         timeout_seconds=float(os.environ.get("USER_ENGINE_DELIVERY_TIMEOUT", "5")),
+        mail_client_bindings=os.environ.get("USER_ENGINE_MAIL_CLIENT_BINDINGS"),
     )
     app = PortalApplication(
         service,

src/user_engine/service.py

@@ -2964,7 +2964,7 @@ class UserEngineService:
                     deliver(claimed)
                 except Exception as exc:
                     code = getattr(exc, "code", "delivery_unavailable")
-                    if code not in {"mail_unconfigured", "delivery_outcome_unknown", "temporary_deferral", "permanent_rejection", "recipient_suppressed", "provider_timeout", "provider_unavailable"}:
+                    if code not in {"mail_unconfigured", "mail_binding_denied", "delivery_outcome_unknown", "temporary_deferral", "permanent_rejection", "recipient_suppressed", "provider_timeout", "provider_unavailable"}:
                         code = "delivery_unavailable"
                     outcome = replace(claimed, claimed_by=None, claimed_at=None,
                         failed_at=utc_now(), failure_reason=code,

src/user_engine/web.py

@@ -1299,6 +1299,7 @@ class PortalApplication:
         if event.delivered_at: return "Accepted by delivery adapter; receipt by the person is unverified"
         if event.failure_reason == "delivery_outcome_unknown": return "Mail outcome unknown — check provider evidence before sending another message"
         if event.failure_reason == "mail_unconfigured": return "Mail is not configured — use assisted setup or restore the mail lane"
+        if event.failure_reason == "mail_binding_denied": return "Mail is not admitted for this tenant — fix the mail client binding or use assisted setup"
         if event.dead_lettered_at: return "Delivery stopped after repeated failures"
         if event.failed_at: return "Delivery failed; retry pending"
         if event.claimed_by: return "Being processed"