One of our IT Support clients asked us why some emails are coming in with extra HTML content. After some searching, we've discovered that is was due to the MailScanner antispam system: it was parsing emails improperly and adding extra junk. We have decided to solve this by skipping the email from that domain from being scanned altogether.
One way to do so is to change the Scan Messages directive in MailScanner.conf from:
Scan Messages = yes
Scan Messages = %rules-dir%/scan.messages.rules
Then, in your rules directory, create a file called scan.messages.rules. In that file, be sure to add the line for scanning all by default: