Email, MX Records and Spam

One of the benefits of owning your own domain is the ability to experiment.

Because I've gone back and forth between email hosts for several times I realized last week I actually have 3 active email hosts. Fastmail, Hover and the Google Apps account I setup years ago when it was free for less than 50 users.

Since switching to Fastmail full time I've been getting a ton of spam. Way more than I ever got with Hover or Google. With that in mind I ran a test. I switched email hosts 3 times in a week using each host for 2 days, and the results I got are surprising:

Fastmail - 15 to 20 spam emails a day NOT caught by their spam filter plus another 10 or so that were.

Google - only 10 to 15 spam emails, all caught

Hover - only a couple slipped through, the other 10 or so were caught

Here's what is so surprising, the spam I got at Fastmail continued to come in to that account (I checked via the webmail access) even after I changed the MX records to point to Google. Hell, even after I changed them again to Hover. How is this even possible? There were no MX records for any domain address pointed to Fastmail. Yet they were all addressed to my main contact info. Also, the spam received when I was hosting at Hover and Google were basically the same. Slightly different subjects and senders, but obviously the same scam. The spam that Fastmail didn't catch never showed up at either of the 2 hosts. They're all more or less the same email but they only seem to appear when I'm hosting at Fastmail, and their filter NEVER catches them.

I'm stumped.

I don't want to say anything nefarious is going on at Fastmail. Enough geeky folks I respect swear by them, but this is a little alarming.

Any ideas?