Add email.tlsname config option

The existing `email.smtp_host` config option is used for two distinct
purposes: it is resolved into the IP address to connect to, and used to
(request via SNI and) validate the server's certificate if TLS is
enabled.  This new option allows specifying a different name for the
second purpose.

This is especially helpful, if `email.smtp_host` isn't a global FQDN,
but something that resolves only locally (e.g. "localhost" to connect
through the loopback interface, or some other internally routed name),
that one cannot get a valid certificate for.

changelog.d/17849.feature

@@ -0,0 +1 @@
+Added the `email.tlsname` config option.  This allows specifying the domain name used to validate the SMTP server's TLS certificate separately from the `email.smtp_host` to connect to.

docs/usage/configuration/config_documentation.md

@@ -673,8 +673,9 @@ This setting has the following sub-options:
    TLS via STARTTLS *if the SMTP server supports it*. If this option is set,
    Synapse will refuse to connect unless the server supports STARTTLS.
 * `enable_tls`: By default, if the server supports TLS, it will be used, and the server
-   must present a certificate that is valid for 'smtp_host'. If this option
+   must present a certificate that is valid for `tlsname`. If this option
    is set to false, TLS will not be used.
+* `tlsname`: The domain name the SMTP server's TLS certificate must be valid for, defaulting to `smtp_host`.
 * `notif_from`: defines the "From" address to use when sending emails.
     It must be set if email sending is enabled. The placeholder '%(app)s' will be replaced by the application name,
     which is normally set in `app_name`, but may be overridden by the
@@ -741,6 +742,7 @@ email:
   force_tls: true
   require_transport_security: true
   enable_tls: false
+  tlsname: mail.server.example.com
   notif_from: "Your Friendly %(app)s homeserver <noreply@example.com>"
   app_name: my_branded_matrix_server
   enable_notifs: true

synapse/config/emailconfig.py

@@ -110,6 +110,7 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             raise ConfigError(
                 "email.require_transport_security requires email.enable_tls to be true"
             )
+        self.email_tlsname = email_config.get("tlsname", None)
 
         if "app_name" in email_config:
             self.email_app_name = email_config["app_name"]

synapse/handlers/send_email.py

@@ -71,6 +71,7 @@ async def _sendmail(
     require_tls: bool = False,
     enable_tls: bool = True,
     force_tls: bool = False,
+    tlsname: Optional[str] = None,
 ) -> None:
     """A simple wrapper around ESMTPSenderFactory, to allow substitution in tests
 
@@ -88,9 +89,13 @@ async def _sendmail(
         enable_tls: True to enable STARTTLS. If this is False and require_tls is True,
            the request will fail.
         force_tls: True to enable Implicit TLS.
+        tlsname: the domain name expected as the TLS certificate's commonname,
+           defaults to smtphost.
     """
     msg = BytesIO(msg_bytes)
     d: "Deferred[object]" = Deferred()
+    if tlsname is None:
+        tlsname = smtphost
 
     def build_sender_factory(**kwargs: Any) -> ESMTPSenderFactory:
         return ESMTPSenderFactory(
@@ -117,10 +122,10 @@ def build_sender_factory(**kwargs: Any) -> ESMTPSenderFactory:
     else:
         # for twisted 21.2 and later, there is a 'hostname' parameter which we should
         # set to enable TLS.
-        factory = build_sender_factory(hostname=smtphost if enable_tls else None)
+        factory = build_sender_factory(hostname=tlsname if enable_tls else None)
 
     if force_tls:
-        factory = TLSMemoryBIOFactory(optionsForClientTLS(smtphost), True, factory)
+        factory = TLSMemoryBIOFactory(optionsForClientTLS(tlsname), True, factory)
 
     endpoint = HostnameEndpoint(
         reactor, smtphost, smtpport, timeout=30, bindAddress=None
@@ -148,6 +153,7 @@ def __init__(self, hs: "HomeServer"):
         self._require_transport_security = hs.config.email.require_transport_security
         self._enable_tls = hs.config.email.enable_smtp_tls
         self._force_tls = hs.config.email.force_tls
+        self._tlsname = hs.config.email.email_tlsname
 
         self._sendmail = _sendmail
 
@@ -227,4 +233,5 @@ async def send_email(
             require_tls=self._require_transport_security,
             enable_tls=self._enable_tls,
             force_tls=self._force_tls,
+            tlsname=self._tlsname,
         )