Hi, i had the same problem here with Mozilla Thunderbird after the update to version 4.5.x???
This is what i found in the FAQ:
Q: After updating to version 4.5 I can no longer send/receive emails.
A:
The following text applies to Windows NT/2000/XP/2003 systems only.
Mail scanning in avast 4.5 is implemented differently than in previous versions. In avast 4.5, mail filtering works transparently. That is, no reconfiguration of mail clients of any kind is required for the mail scanning function to work.
If you have, in the past, manually reconfigured your e-mail client program to route all traffic through the avast! Internet Mail provider (by changing the server addresses to 127.0.0.1 and the username value - as described in the avast help files), you should remove these extra settings (i.e. revert them to the original state; in other words, have the accounts set up exactly as if there was no avast installed). This will allow the avast transparent mail proxy to be used.
And it worked for me
edit: hmmm didn't read that well, you already tried