warn users if SL is used with another forwarding service

app/config.py

@@ -317,6 +317,8 @@ ALERT_SEND_EMAIL_CYCLE = "cycle"
 
 ALERT_NON_REVERSE_ALIAS_REPLY_PHASE = "non_reverse_alias_reply_phase"
 
+ALERT_FROM_ADDRESS_IS_REVERSE_ALIAS = "from_address_is_reverse_alias"
+
 ALERT_SPF = "spf"
 
 # when a mailbox is also an alias

email_handler.py

@@ -86,6 +86,7 @@ from app.config import (
     ALERT_HOTMAIL_COMPLAINT_REPLY_PHASE,
     OLD_UNSUBSCRIBER,
     ALERT_NON_REVERSE_ALIAS_REPLY_PHASE,
+    ALERT_FROM_ADDRESS_IS_REVERSE_ALIAS,
 )
 from app.db import Session
 from app.email import status, headers
@@ -133,7 +134,6 @@ from app.errors import (
     VERPTransactional,
     VERPForward,
     VERPReply,
-    MailSentFromReverseAlias,
     CannotCreateContactForReverseAlias,
 )
 from app.log import LOG, set_message_id
@@ -2084,10 +2084,10 @@ def handle(envelope: Envelope, msg: Message) -> str:
     )
 
     # region mail_from or from_header is a reverse alias which should never happen
-
+    email_sent_from_reverse_alias = False
     contact = Contact.get_by(reply_email=mail_from)
     if contact:
-        raise MailSentFromReverseAlias(f"{contact} {contact.alias} {contact.user}")
+        email_sent_from_reverse_alias = True
 
     from_header = get_header_unicode(msg[headers.FROM])
     if from_header:
@@ -2098,9 +2098,20 @@ def handle(envelope: Envelope, msg: Message) -> str:
         else:
             contact = Contact.get_by(reply_email=from_header_address)
             if contact:
-                raise MailSentFromReverseAlias(
-                    f"{contact} {contact.alias} {contact.user}"
-                )
+                email_sent_from_reverse_alias = True
+
+    if email_sent_from_reverse_alias:
+        LOG.w(f"email sent from reverse alias {contact} {contact.alias} {contact.user}")
+        user = contact.user
+        send_email_at_most_times(
+            user,
+            ALERT_FROM_ADDRESS_IS_REVERSE_ALIAS,
+            user.email,
+            "SimpleLogin shouldn't be used with another email forwarding system",
+            render(
+                "transactional/email-sent-from-reverse-alias.txt.jinja2",
+            ),
+        )
 
     # endregion
 

templates/emails/transactional/email-sent-from-reverse-alias.txt.jinja2

@@ -0,0 +1,15 @@
+We have detected that SimpleLogin is probably used along with another forwarding email service.
+
+Please note that the following use cases aren't supported by SimpleLogin:
+
+- your mailbox is an email alias (i.e. not a "final" email address)
+- your mailbox automatic forwarding enabled
+- your alias receives automatic forwarding email from another mailbox
+
+As different email forwarding systems are usually incompatible, using several forwarding services can cause issue
+with email delivery.
+
+Please let us know if you have any question.
+
+Best,
+SimpleLogin team.