The email scanning isn't a supplement to an AV but an integral part of it. avast scans incoming email before they are saved in your inbox, etc. So it has time to alert you and effectively block it being saved in your inbox, which some AVs have difficulty removing an infected email from an email database file.
If you receive non-SSL emails yes avast will scan them, for the SSL emails they aren't scanned, but this applies to all emails - never click on links or open attachments in unsolicited emails. If the email contains an attachment and you believe the email legit, save the email attachment to your hard drive.
Depending on the file type, .exe, .com, etc. avast would scan it automatically but you can also scan it manually, by going to where you saved it too, right click on it and select Scan selected areas for viruses.
If you set the Internet Mail provider to High it can detect multiple identical outbound emails in a period of time. This could provide an early indication of a hidden or undetected spambot, which you can get help with.