If you run your own MDA, you could do a lot worse than to filter incoming mail against https://www.spamhaus.org/zen/; the free tier is extremely broad, and the coverage is excellent -- I get maybe one spam email a week, tops.
I'm also happy with Spamhaus Zen generally, but spam sent via Gmail is one category of email it won't stop, since it only blacklists pure spam sources, and Gmail is obviously a legitimate mailhost. I get a steady trickle of spam from Gmail addresses, mostly SEO-consultant spammers who grabbed my email from the whois info.
Whenever they solicit me I respond immediately with links to my own SEO articles. Of course the very best SEO is for ones own website to go viral at an SEO board.
One enterprising young South Asian was obviously a clueless newbie so I gave my reply a great deal of care. A few hours later he responded with:
"You are my SEO master."
I dontbreally offer SEO consulting but to claim that I do results in lots of Google Juice.
