Tor is a kind of peer-to-peer function that provides for anonymizing of network access by routing transactions through a network of "servers" that donate bandwidth for that purpose.
A short scan shows that there are recommendations to those donating bandwidth that useful ports they can define for accessing the servers include (preferably 443) but also 143 and 110. I assume because these ports are "well known" and not frequently blocked and the folks acting as servers probably do not run mail servers at the same time.
As David mentioned, by default, avast! intercepts calls to servers on ports 25, 110 and 143 (SMTP, POP3, IMAP). Avast! assumes that any such calls are intended for mail purposes.
It certainly has nothing to do with any update to avast!
The best solution will probably be to exclude the Tor process in the [MailScanner] section of the avast4.ini file.
