Short answer No leave the email scanning options enabled.
Slightly longer answer, avast works well with the various incarnation of MS email clients, OE, Windows Live Mail, Windows Mail, etc. Not to mention the avast email scanner works differently to many scanners as it scans incoming email in a localhost proxy (before it even gets to your inbox) so it isn't working in your mail boxes so can't corrupt files that it isn't working with.
I would dispute the "The email scanning option is a redundant feature" comment, email is still a major route of entry for malware.
So leave it enabled and I would go a step further and increase the Internet Mail provider's sensitivity to High. On High it can detect multiple identical emails in a period of time, this could be an early indication that there is a hidden or undetected trojan spambot on your system.
I hate people that make these kine of blanket statements for 'all' AVs as they aren't all the same.
The most dangerous type of scan isn't the inbound or outbound scan but an on-demand scan of your HDD when your email folders (database files) are scanned. If an infected email is found inside one of these some AVs could corrupt the database file when trying to extract an infected email from within it or worse still delete the file as it would treat the infection as one file.
So care has to be exercised in the on-demand scans if a detection is pointing to your email files and seek advice. avast hasn't had any problems extracting infected emails from within the OE .dbx files in the past and I don't know if this is the file format retained by Windows Mail.
This statement is cr*p too:
These email scans can also cause problems with opening attachments in Windows Mail to."
You should 'never' open an attachment directly from an email (and that is the message they should be promoting), but save it to your HDD, where you can svan it in safety before opening it.
God I'm mad at these blanket statements.