Duane Raymond writes:
Regarding emails not reaching some users, the logs seem to indicate the emails are being sent fine and in communication with users it isn't in spam or bulk emaIL folders. Some emails arrive hours after despite being sent on time.
This may be greylisting, the practice of sending an artificial temporary SMTP failure in hopes that spammers won't come back but a legit MTA will automatically retry. IIRC the traditional first retry comes after 4 hours, although most greylisters will allow the transaction after 15 minutes. I think modernized sites do retry a lot quicker, but I don't know what MTA defaults are nowadays.
If the mail is not being delivered direct from Mailman's MTA to the recipient's MTA, you wouldn't be able to tell from the sender and recipient times alone, you'd need the whole Received trace from the recipient's copy to find the point where the delay is happening.
Steve