0x800ccc0f Outlook POP3 - The connection to the server was interrupted

I recently ran into a strange issue with POP3 and register.com email accounts for my domains using Outlook 2010. The following explains the issue and the solution.

I had a total of five email accounts set up in Outlook 2010.  All five for domains at register.com.  Two of the accounts worked just fine sending and receiving email.  The other three kept getting this message:

The connection to the server was interrupted.  If this problem continues, contact the server administrator or Internet service provider (ISP). The server responded: -ERR

I spent a few hours yesterday figuring out what the specific issue was.  Spoke with register.com first to confirm it wasn't them.  Believe it or not, their support was quite helpful and the wait time minimal.  After confirming that my accounts worked on other machines with Outlook 2010, I started looking for other clues.  I ultimately was able to confirm that the issue was with my anti-virus software Avast 5.0.  I simply started disabling services temporarily and retesting the incoming mail in Outlook after each service was disabled.  After disabling the Mail Scanner, bingo.  Next step was to carefully review each of the settings in the real time mail scanner.  Looking even closer, I confirmed that a setting in the SSL Account section was the culprit.

Avast had somehow placed an entry for register.com and set its encryption setting to TLS.  I didn't need that and opted to switch it to None.  Immediately, my other three email accounts worked just fine.   I cannot tell you why Avast let the first two accounts through with the exact same settings as the other three.  That remains a mystery.

Even if you don't use Avast, your anti-virus software may have similar capabilities and similar action steps to execute.  If you run into the same problems I did, I suggest checking your anti-virus first.