Prevent abuse of sendmail for spam

Mail will be marked as spam by major ISPs (including webmail providers like gmail, hotmail, yahoo) for several possible reasons:

1. If you're sending it from a residential IP address
2. If you're sending it from an IP address with a poor reputation
3. If you're sending mail which matches certain patterns (these are hard to describe, but software looks for things like "Congratulations, you've won $1 billion!", in a fuzzy-matching sort of way).
4. If you send too much mail to the ISP too fast
5. If too many people at the ISP click the "This is spam" button on your emails
6. If you don't use SPF to identify which mail servers for your domain may send email, and which servers may not
7. If you don't use DKIM to sign your messages
8. If you haven't requested permission to be a "bulk sender" (some offer this like AOL and hotmail)
9. If you IP address is on any DNS blocklists

and many, many other possible reasons.

You can check the reputation of your IP address at https://www.senderscore.org/
You can check if you're on various blocklists at http://www.mxtoolbox.com/blacklists.aspx

If you want to run MIMEDefang together with sendmai you can do most of what you write by providing the appropriate routines in mimedefang-filter. However, your best approach would be to use SpamAssassin first and then see what more filters you can implement yourself. SpamAssassin can be executed via MIMEDefang which means you can combine it with custom rules of your own.