I use Foxmail which is not supported by default by avast (which is also the case for opera). You just have to be sure that the E-mail scanner service is running for each user : you have to loggin as each user. For each e-mail account the pop and smtp servers have to be set to 127.0.0.1.
For the accounts in your e-mail software :
POP : account#pop.myserver.com
if you have smtp authentication :
SMTP : account#smtp.myserver.com
if not :
SMTP : #smtp.myserver.com

For more details see “Setting up the mail protection” and “Manual setting of mail protection”.

Never use Outlook !!