We are trying to send email with over 8 MB attachment . The mail server is Zimbra. The message stays in the outbox and is not delivered while attachments lower than 8 MB are delivered. We have another issue where the mails are not delivered at all from a particular IP or ISP.
We see below in the mail logs:
timeout after data ( 0 bytes ) from unknown.
milter-reject mail from unknown service unavailable try again later.
We have gone through forums and they indicate MTU for timeout error and postconf milter_default_action is set to tempfail for milter-reject which should be set to accept.
Please let us know the probable cause and the way to fix the above errors.