I've seen an increase in spam recently, and a lot of it has included .html attachments. Some emails masquerade as bounce messages, encouraging users to open the HTML to see what message bounced. Others use similar techniques to trick the user into opening the file. I'm considering blocking all email with a .html file attached, in an attempt to catch more of this spam. Using zen.spamhaus.org had greatly reduced the amount of spam received when I started using it last September, but a lot of these .html attachments have been getting through.
I took a look inside a few and they used META REFRESH tags to redirect to some website. Since the attachment is base64 encoded, SpamAssassin can't check the URL against the various URL blacklists.
I'm using SpamAssassin, ClamAV and simscan (an add-on for qmail) on my system, and can easily block certain attachments. My question is whether .html files are commonly sent as attachments on legitimate mail.
NOTE 1: I'm not referring to HTML formatted email, I'm specifically talking about a .html file as an attachment that must be opened in a web browser.
NOTE 2: I'm considering this for the hosting servers I maintain for 150+ domains. Most are small businesses with 1-5 accounts.