Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Account: '****.com', Server: '****.com', Protocol: POP3, Port: 110, Secure(SSL): No, Socket Error: 10053, Error Number: 0x800CCC0F
At first I figured it wasn't a big deal because the next time the user clicked the "send/receive" button it would continue to download e-mail just fine. Then I got a complaint that an e-mail that had a spreadsheet attached wasn't received. After having the sender try multiple times to resend the message it was clear this was a more serious problem because this error was indicating an e-mail wasn't being received rather than just some sort of anomalous network error or timeout error. What was even worse was that the e-mail was being deleted from our hosting server so we didn't even have a backup copy of the message we could view via webmail!
I tried many different configuration settings and nothing was working. Finally, I decided to take a look at my firewall appliance. We are using a SonicWALL firewall appliance and it works great. Unfortunately, I think it downloaded an update that added a small configuration setting to my incoming POP e-mail. In the "Gateway Anti-Virus" configuration screen I had POP3 virus scanning enabled. This was working fine until the following setting appeared "Restrict Transfer of MS-Office type files containing macros (VBA 5 and above)". This was preventing certain Excel spreadsheets and Word documents from being received. Unfortunately, rather than allow the message to come through without an attachment, it was preventing the entire message from coming through at all and causing Outlook to display a strange connection error message. Even worse it was causing the hosting server to delete the message entirely! I decided to remove the POP3 scanning entirely to prevent something like this from happening again.