A colleague in South Florida has a myacc.net email account provided by Advanced Cable Communications (her cable company). After experiencing no problems sending me email over several months, for the past two weeks all her emails to me have bounced back to her with a message from the mail.myacc.net mail server that says "mail for imacs.org loops back to myself".
Neither she nor I have been able to contact any tech person at myacc.net with a view to asking them what might be causing this behavior. In particular, emails to email@example.com go unanswered, even when I provide a ReplyTo address that is not on the imacs.org domain.
I have checked out our Zimbra configuration as best I can using tools such as those at MX Lookup Tool - Check your DNS MX Records online - MxToolbox, but no configuration problems have surfaced.
Does anyone have any suggestions as to what might be causing this behavior? As far as I know, this is the only mail server that behaves this way when serving email intended for imacs.org addresses.