[mdpm]

uh, nope. Email validation should check there's a valid MX record for the domain in question, and leave it at that.

[Luyt]

Or a valid address record. Mailers fall back to 'A' records in the absence of a 'MX' record.

[mdpm]

yes, and we could continue into checking something is actually listening for mail there, etc. But you get my point :)