add retry mechanism when sending email to new user

ceod/transactions/members/AddMemberTransaction.py

@@ -1,3 +1,5 @@
+import smtplib
+import time
 import traceback
 from typing import Union, List
 
@@ -89,12 +91,25 @@ class AddMemberTransaction(AbstractTransaction):
         # The following operations can't/shouldn't be rolled back because the
         # user has already seen the email
 
-        try:
-            self.mail_srv.send_welcome_message_to(user, password)
-            yield 'send_welcome_message'
-        except Exception as err:
-            logger.warning('send_welcome_message failed:\n' + traceback.format_exc())
-            yield 'failed_to_send_welcome_message: ' + str(err)
+        # sometimes Postfix says 'User unknown in local recipient table' if we try
+        # to send an email to a user right after they were created
+        max_tries = 3
+        for i in range(max_tries):
+            try:
+                self.mail_srv.send_welcome_message_to(user, password)
+                yield 'send_welcome_message'
+                break
+            except smtplib.SMTPRecipientsRefused:
+                if i < max_tries - 1:
+                    logger.warning('SMTP recipient refused, sleeping and trying again')
+                    time.sleep(3)
+                    continue
+                else:
+                    yield 'failed_to_send_welcome_message: recipient refused too many times'
+            except Exception as err:
+                logger.warning('send_welcome_message failed:\n' + traceback.format_exc())
+                yield 'failed_to_send_welcome_message: ' + str(err)
+                break
 
         # don't subscribe club reps to csc-general
         if self.terms: