Occcasionally, I have a user who receives a pop-up warning about Proxy Error 1003,11 (Your email message was unable to be sent because your mail server rejected the message). . That particular message still went through, but sometimes messages don't go through, although they appear to be sent and the message is moved to the user's "Sent" folder.
This is using POP mail, no Exchange server involved. Happens with Outlook 2003 and 2007.
I can't figure a way to prevent the 1003,11 message, or to ensure that the client is always able to send mail. In SEPM, I have disabled email scanning and Outlook protection, but the problem still exists with certain clients. It is possible that attachments may be involved, but I'm not sure if this is true in every case.
We have been using Endpoint Security 11 with about 25 clients for about a month now. Previously, we had been running Symantec Antivirus Corporate Edition, but even as long as a year ago, the problem would occasionally appear.