in reply to
Re^3: Determine if domain is actually used for email
in thread Determine if domain is actually used for email
I have no idea where UDP came into the conversation since SMTP is a TCP protocol. I agree that there is no way to be sure unless you can deliver mail to a recipient and get a DSN or, preferred, a response. Seeing if the server is listening on port 25 and is responding in a manner consistent with a SMTP server and not giving a 521 status code is a pretty safe bet though.