Spamming Without DNS

I would like someone more knowledgeable perhaps explain this.

I have an email address on my own domain that has existed for about 4 years now. About 5 weeks ago, the VPS it were on died and so the email went unresolved for about 3 weeks. I then transferred to a new VPS, and set up the email. About a week later, I moved again.

I left up the 2nd VPS for DNS propagation to take place. What interest me is that the server is still running about a week later, and the email server is still getting spam messages directed to it. So somehow, the spams are being send to the old IP, with a valid recipient, even when the most stubborn of DNS cache should have updated by now.

So it seems (some) spammers are just capturing email address and the IP for the server, storing it, and mass spam straight to those IPs instead of resolving the server.