Mail remains in exchange queue with Avast 8 installed.
Problem arises is ESP is using self signed certificates.
Checking %ALLUSERSPROFILE%\AVAST Software\Avast\log\Mail.log you’ll find
Cannot establish TLS with SMTP server A.B.C.D:25, SSL_connect error 1:error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
Resolution
Remove the mail server / domain from the list which your exchange server is having problems connecting to.
The message at the bottom leads to me believe the problem may only arise if site was upgraded from Avast 7 to Avast 8?