Just installed avast and got a worrying popup. It stated:
avast! has detected a secure connection from your mail program (thunderbird.exe) to the IMAP server 74.208.5.19 (perfora.net)
I have 2 IMAP accounts set up in TB, but none use that site. Before today I have never heard of that site, and going to perfora.net gives a permissions error
Avast needs to take care of the secure connection to your email server so the mail shield can scan your mail.
Please read the following to make sure you set up your accounts properly.
From avast knowledgebase → https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=842
Post similar to yours → http://forum.avast.com/index.php?topic=101219.0