I have been having the same problem, using Eudora and Outlook Express on WinXP Pro. I have not checked Outlook yet as I do not normally use it but for Eudora I disabled the SSL both incoming and outgoing on all accounts and it seems to be working. Avast claims it provides the SSL. (Eudora calls it Secure Sockets, I set all to Never)
I have several email accounts set up in Eudora (yeah, some people still use Eudora) and found that if even one account has SSL enabled for SMTP or POP it breaks Avast and I can no longer check mail on any email account.
When Avast (free) breaks like this I have had to go into Avast, select REAL-TIME SHIELDS, then Mail Shield, then Expert Settings, then SSL accounts. There I have to delete the account before I can receive mail again. I find that when this happens Avast changes the security from SSL to TLS for some reason. Maybe this is a bug? I don't know
This only started for me today, after Avast updated to the 5.0.677 software. I called the Avast free tech support, provided by iYogi, and was on the phone with the tech remote controlling my computer for about 45 minutes. He claimed it was a problem with my computer and had nothing to do with Avast. He offered to fix it for $175. I would not recommend calling them.