KB0808271 Intermittent problems sending mail from clients
Messages are sometimes not getting from the User's email client (say, Outlook) to Mailtraq, or are repeatedly sent by the client to Mailtraq.
You may find that this affects all mail from a particular user, or all mail of a certain type - say with attachments, or attachments of a certain type, or over a certain size, or to multiple recipients.
This will happen if the connection between the client and Mailtraq is broken during the transmission of the message. If the break is at the beginning of the transmission then it appears that the message is not sent, if it happens right at the end but before Mailtraq acknowledges completion then Mailtraq thinks it has the message and so sends it on, but the client believes the send to have failed and so sends the message again.
Breaks in communication of this nature are nearly always caused by misconfigured or inappropriate anti-virus software or a misconfigured firewall.
It is important to remember that all active 'protection' software changes over time as it receives updates. Some updates make substantial changes in the rule sets and assumptions, so their behavior may change even though there has been no manual change in the configuration.
Common issues are that the anti-virus software sees 'excessive' SMTP traffic on Port 25 between the client machine and the server and mistakenly thinks that a Trojan or other malware is in play, or that the encoding of a document attached to an email - say, a PDF - trigger the break in the connection.
They can also be caused by network failures caused by wiring faults, failing switches and routers, and network interface cards (NICs). Hardware causes are comparatively rare, and of these, NIC failure is the most common cause.
First - ensure that your anti-virus implementation on the mail server follows the recommendations at Mailtraq anti-virus
Then, adjust your anti-virus and/or firewall software to allow traffic between the client and Mailtraq.
Your anti-virus or firewall vendor can advise you how to do this.