Some 3rd party email clients such as Thunderbird and the one I use "The Bat" have problems with the latest version of Avast a/v (ver. 8).

I got a notice that my trusted root certificate wasn't found when I checked my Gmail POP3 accounts from The Bat.

Here's what others have said in Avast's forums..

And a link from that thread goes to a fix here...

the Mail Shield stores its Root certificate to the Windows cetificate store which is used by most mail clients. Some mail clients (e.g. Thunderbird) use its own certificate store which needs additional handling.