Your Mother,
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.

Cheers - L~R

In reply to Re^4: Determine if domain is actually used for email by Limbic~Region
in thread Determine if domain is actually used for email by vit

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