Without sending mail to the address and seeing whether it bounces (and even then you face the halting problem), you cannot determine whether an email address is valid. Even if you apply the email header standard, you can have problems, because there are deliverable addresses that aren't RFC-822 (the mail header standard) compliant, and addresses that aren't deliverable which are compliant.
Many are tempted to try to eliminate many frequently-invalid email
addresses with a simple regexp, such as
/^[\w.-]+\@+\w+$/. However, this also throws out many valid ones, and says nothing about potential deliverability, so is not suggested. Instead, see http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz , which actually checks against the full
RFC spec (except for nested comments), looks for addresses you may not wish to accept email to (say, Bill Clinton or your postmaster), and then makes sure that the hostname given can be looked up in
DNS. It's not fast, but it works.