You can use the 'Mail Protection Wizard' in avast 4.1 for IncrediMail.
Here is an example for Opera and avast 4.1:
Go to Account properties, page Servers.
Under Incoming POP server
Server: pop.example.com -> 127.0.0.1
Username: yourname -> yourname#pop.example.com
Secure connection: NO
Under Outgoing SMTP server
Server: smtp.example.com -> 127.0.0.1
Username: empty -> #smtp.example.com
Password: empty -> arbitrary, non empty
Secure connection: NO
Open the file C:\Program Files\ALWIL Software\Avast4\Data\avast4.ini, find section [MailScanner], add these lines under it and save the file.
DefaultPopServer=pop.example.com
DefaultSmtpServer=smtp.example.com
UseDefaultSmtp=0
You can then start the 'Mail Protection Wizard' and choose 'Set up the protection manually' to set additional options and to enable automatic start of the mail scanner service.