Don't validate the address beyond looking for an '@' in the string; just send an email with a confirmation link to see if it actually works.
For reference, I didn't try any of the more obscure email address features, such as comments or quotation. My address simply has the form firstname@lastname.email
Please don't enable anyone to just send emails to arbitrary email-addresses through you. That's how your email/domain will quickly get marked for SPAM.
At the very least, put in place a rate-limit per email-id (and source-ip too maybe). Otherwise i can simply keep entering email-ids of people to spam.
^[^@]+@[^.]+\..+$