[Mailman-Users] list stop sending - no changes to server or mailman in months

Mailman's 'smtp' and 'smtp-failure' logs will tell you about delivery
to the outgoing smtp server. If there is an entry in the 'smtp' log of
the form '%(msg_message-id)s smtp to %(listname)s for %(#recips)d
recips, completed in %(time).3f seconds' and no entry in the
'smtp-failure' log, Mailman has successfully transferred the message
to the outgoing MTA. If bounces are subsequently returned, they will
be noted in the 'bounce' log if bounce processing is enabled.

As far as how Mailman delivers, this depends on a number of things. The
basic controls are Defaults.py/mm_cfg.py options SMTP_MAX_RCPTS
(default 500) and SMTP_MAX_SESSIONS_PER_CONNECTION (default 0). These
defaults say messages should be delivered to the MTA in chunks with a
maximum of 500 recipients per chunk and as many chunks as necessary
with one connect to the server. SMTP_MAX_RCPTS adjusts the chunk size,
and if SMTP_MAX_SESSIONS_PER_CONNECTION is > 0, after that many chunks
are sent, Mailman will close the SMTP connection and re-open it for
the next chunk.

VERP and personalization will reduce the chunk size to 1 - i.e., each
recipient will be sent to individually.

In any case, there will be only one 'smtp' log entry for a successful
message send indicating the total #recips and time, regardless of how
many chunks are sent.

See the Defaults.py/mm_cfg.py settings for VERP_DELIVERY_INTERVAL and
OWNERS_CAN_ENABLE_PERSONALIZATION and the list's personalize setting
if OWNERS_CAN_ENABLE_PERSONALIZATION = Yes.

As far as resolving the issue, the problem seems to be out of Mailman's
hands based on what you say about the post and smtp logs. You may be
able to work around the issue by mm_cfg.py settings like for example

SMTP_MAX_RCPTS = 5
SMTP_MAX_SESSIONS_PER_CONNECTION = 1

but this may not help depending on what the actual problem is.