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Quick answer: You can't.
Beyond basic syntax checking, which "firstname.lastname@example.org" would pass, there are tools to check the existence of the domain and the presence of a mail server via DNS. But some spammers have this figured out and will send you "email@example.com". Due to spammers again, most email servers have email account verification services blocked so the only way to validate an email address on a valid email server is to send a message and wait for a user response.
My solution would be to make spamming a capital offense, but I think I would meet some resistance promoting this line of defense!
> We have a feedback form which has been getting used by spammers and entering
> email addresses such as firstname.lastname@example.org. How can I check that the email
> entered into the form is a valid email address?
as steve says, you can't really but you can make it harder by using captcha or
simply popping up the feedback form via js like ray's blog (that defeats a
significant amount of this kind of goop).
yeah we also have captcha code implented, but someone is still getting through. I can have this setup so that only university community can access the form, but that closes the form out to visiting profs using our smart classrooms.
I did think about checking the addresses against a db table which already contains email addresses that I manually blocked from our listserv.