RE: Editing E-Mail Addresses -- RPG400-L

Actually this went around the list a month or two ago. IIRC, you check
for an MX record first. If it exists you MUST use it. If is doesn't
exist, you SHOULD check for an A record w/the domain name. If it exists
you use it. If no A record exists then you're toast.

That's not precisely true. If a CNAME record exists, then that mightpoint to a different A record. It's possible (though, discouraged) toroute e-mail using a CNAME record.


So, technically, this is the process:

a) Look up the MX records. If one exists, make sure it points to a validA record (or a CNAME record that points to an A record) as long as one MXrecord is valid, the domain is good.

b) If no MX records exist, try looking up an A record for the originaldomain. If that works, the domain is good.

c) If no A record exists, try looking up the CNAME record. If that works,resolve the A Record that the CNAME record points to. If that works, thedomain is good.

This leaves you with a process where if a domain lookup for any of the A,CNAME or MX records works, the domain is good. At that point, it doesn'tmatter which order you do them in, because ANY of them being correct willmean that the domain is correct.

And that's what my program (in the article you referenced) does. That'sEXACTLY what it does.

Now, if you're going to actually CONNECT to these, then the order of thelookups becomes important. My program doesn't do that, so it doesn'tmatter.

If you're assertion is that "test.com" shouldn't be valid because e-mailfor test.com is supposed to be sent to "testcentral.com", neither Walden'smethod nor mine will detect this. The only difference between the twomethods, is that I checked the A and CNAME records first instead ofsecond. No matter what order you check them in, it'll still think"test.com" is valid, because the A record lookup for TEST.COM will stillwork.

The only way you could ACTUALLY tell that test.com isn't a valid domainfor e-mail is if you tried actually connecting to TEST.COM as an SMTPserver. And that's not foolproof either, because if the SMTP serverhappens to be down at the moment, or if traffic is too high going to it,it might not respond in spite of being a valid e-mail address. An actualSMTP server will queue up the message and keep retrying for a few days,but you obviously can't do that in the case of verifying that the domainis valid.

So I don't think you'll be able to solve the problem of test.com vs.testcentral.com.