Thanks for your comments, Thomas.

I don't think caching would be worthwhile in my case, because:
1. Each address will rarely be checked. It's basically a check to make sure a subscriber's address is valid when it's entered into my database.
2. The status of validity could change as domains disappear, etc (though this would also be rare).

However, I'm interested in what you're saying about talking to the receiving mail server.  Are you able to point me in the right direction for that kind of coding, please?

tel2