avast cannot scan any email port that is using a secure connection (SSL or TLS). That is, after all, the whole point of secure connections that nobody and no system can look into them. If you attempt to make avast do so you will not succeed and will, in almost all cases, cause avast to prevent the connection being made.
For the present there is a third party free program (STunnel) that can manage your secure connections within your system and pass the resulting mail stream to avast to be scanned - all entirely securely within your system. It takes a little work to download and setup STunnel but there is plenty of helpful guidance available from the Search function in this forum.
If you can wait a bit longer the avast team have advised that the management of the secure connections for which STunnel is currently needed will be built into the next major release of avast expected in the first half of 2009.
For non-secure connections using ports other than the "well known" ports 110 for POP, 25 for SMTP and 143 for IMAP then those ports made be added to the appropriate boxes in the "Redirect" tab of the Internet Mail provider - remembering to use a comma as the separator between port numbers.
