Mailman: make a new SMTP connection for every session

Should allow us to load-balance outbound email better among the
available mail-frontend servers, in combination with the mailman
RR patch.

roles/mailman/templates/mm_cfg.py.j2

@@ -127,6 +127,10 @@ SMTPHOST = 'mail-frontend.{{ domain }}'
 SMTPPORT = 10025
 SMTP_HELO_HOST = '{{ shard_id }}.mailman.{{ domain }}'
 
+# Establish a new SMTP connection for every session, to effectively 
+# load-balance outbound email among available mail-frontend servers.
+SMTP_MAX_SESSIONS_PER_CONNECTION = 1
+
 DEFAULT_ARCHIVE = Off
 DEFAULT_ARCHIVE_PRIVATE = 1
 ARCHIVE_TO_MBOX = 2