Well I thought that avast used the StartTLS option (for the means of starting a secure connection) in the previous avast 6&7 versions.

As an avast user, I don’t know if anything has changed in that regard as there are many that use IMAP on port 143 (gmail users). I don’t know there is anything that can be done manually such as editing the EmailShield.ini file in relation to the connection method. There are possibilities for adding custom ports, e.g. non-standard ports (but 143 is a standard IMAP port) and even then I don’t believe it includes adding a connection method.

I would suggest that you try creating a support ticket as this really needs input from someone on the avast team.