The version 4.5 has a new mail detection that will work automatically. Please, set up your email account just as avast does not exists. It will detect the pop3/smpt/imap ports and scan your messages. Can you make a try and see if it’s ok? Thanks.
New email detection is automatic and do not require user interaction. Anyway, spam killer applications need manually change email account settings 
This is by design.
In 4.1, it was like: mail client > avast > spam killer > mail server
In 4.5, it’s like: mail client > spam killer > avast > mail server