The Firefox browser source code has a list of current TLDs, IIRC. There's over a thousand. You could use that to at least verify the TLD part.
The following may not work for your use case and I'm no expert in the area, but it could be possible to write a simple shell script to dig the mx records, and fail out if there's no such, and if you wanted to go a step further, you could open SMTP, and fail out if you can't, and if you wanted to go a step further you could even on the mx maybe even do a direct ping or some other lookup of the address part in that session, so long as the server is not a catchall. I feel the total bandwidth outlay would be no more then a small http roundtrip. Tho you might get a bit of hit on the connection time, or retries, as my feeling is not all SMTP hosts are speedy. Thanks
