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.

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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.
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.