The Stroumpf,
Protection from e-mail comes in two forms, protection from infected attachments and protection from malicious scripts, applets, and other objects embedded within an HTML formatted message (for example a reference to a malicious web site) that will automatically be processed when the message is read. Generally speaking, a resident e-mail scanner intercepts the message as it's being downloaded and prevents the message, or parts of it, from being saved on your computer, and an on-access scanner scans an infected attachment, script, etc. when you attempt to execute it.
Moving from generalities to specifics about
avast! 4, the
Internet Mail provider has some advanced features that provide additional protection such as checking for multiple extensions or extensions hidden by intervening white space, verifying that a file's type and extension match, and examining message partitions for suspicious properties, to name a few. It also provides extra protections for outgoing mail to prevent worms from spreading by using mass mailing techniques.
The
Standared Shield provider can be configured to scan an infected attachment, script, etc. when you attempt to execute it, write it to disk, or even read it from disk. Since it supports scanning e-mail files, message files (.eml files for example), as opposed to message stores (.dbx files), it will also scan a message when you save it.
Basically, it's a matter of preventing malware from making it to your computer as opposed to preventing it from functioning after it's there and/or going through the hassle of removing it. While there is overlap in the functionality, some consider using an e-mail scanner along with an on-access scanner to be redundant; others consider it a good practice by using a layered defense. I fall in the latter category. The advanced features in the e-mail scanner can protect your computer from becoming infected with malware by alerting you to an e-mail's suspicous characteristics. Some of that malware may not be detected by the on-access scanner if no signature for it is present in the virus database. Hope you find this information useful in deciding what to do.
Regards,
Hornus