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 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.
It's definitely possible to check if the domain is a valid mail server. Give [these instructions](http://www.codeproject.com/KB/validation/Valid_Email_Addresses.aspx) a try.