Email validation MX Lookup

Question

I have been asked to implement some email address validation on a web app - I\'m sure we\'ve all been there a thousand times... however, this time I have been asked to do an MX

Answer 1

are there any edge cases where a valid email address could fail an MX lookup check?

Yes, in that where there is no MX record, MTAs fall back to using the A record instead. So only allowing MX records would fail a class of mail servers that work in the real world.

Allowing any name with an MX or A record at least detects obvious mistypings that result in NXDOMAIN. However it will still allow mistypings that end up at squatter sites. A further step for addresses resolved by A records might be to check where port 25 is accepting connections on that address.