Olivier,
Avast uses these ports since they are pretty much standard in the industry.
In order for Avast to scan email, it monitors the incoming and outgoing ports for SMTP and POP3 which happen to be 110 and 25.
Imap is not used by all computers, but Avast configures itself to monitor the port used by IMAP which is 143.
Not to worry.
Techie