Unfortunately people will sometimes enter an incorrect or invalid
email address in your Form.   Worse, they may enter a valid but
incorrect email address that will deliver to someone who doesn't
want your mail.

Proposed regexps to match email addresses are sometimes posted.
Most of these will fail against perfectly valid email addresses,
like "S=N.OTHER/OU1=X12345A/RECIPNUM=1/MTA-BASIC@attmail.com"
(which is what your address looks like if you are connected to
the Internet via X400 - and if you think that example is too easy,
check the ones at the end of Eli the Bearded's Email Addressing FAQ).

Probably the most complete parser and checker available for download
is Tom Christiansen's, at
http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz
Of course, this still says nothing about deliverability.

A frequently-suggested hack that doesn't work is to use
SMTP EXPN or VRFY commands.   Modern versions of sendmail permit
administrators to disable these commands, and many sites take
advantage of this facility to protect their users' privacy.

Probably the best way to verify an email address is to send mail to
it, asking the user to respond.   Include a clause like "if you have
received this mail in error, please accept our apologies..."