Question: Does anyone check the domain of an email address as part of their verification steps? eg. Confirm that gmail.com exists if the user specified blah@gmail.com as their address.

---

I should note that in my webapp an existing user can forward invites to their friends. I have no need to verify those invites at all. Rather, I just want to warn my user if a typo etc. may be sending an invite to the wrong person. Is this even worthwhile to do?

Answer: 
You could do a dns lookup on the [mx record](http://en.wikipedia.org/wiki/MX_record). Here's an example at Code Project:  

<http://www.codeproject.com/KB/IP/dnslookupdotnet.aspx>

### [addendum]:

That should answer your question as asked, but as a side note I agree with @Franci that the old standby of sending a verification message is better. If someone's done everything else right to fool your validation, you're really not buying much in also checking the domain.
You could execute this command in the console

```
 nslookup -q=mx gmail.com

```

and parse the output for rows containing MX after the hostname

```
gmail.com       MX preference = 40, mail exchanger = alt4.gmail-smtp-in.l.google.com
gmail.com       MX preference = 20, mail exchanger = alt2.gmail-smtp-in.l.google.com
gmail.com       MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
gmail.com       MX preference = 30, mail exchanger = alt3.gmail-smtp-in.l.google.com
gmail.com       MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google.com

```

But I wouln't recommend this kind of check in the first place. If a user doesn't want to give you his e-mail address he will always find a way to trick you.
The only think you can do is send an email with a verification link.