Yep. I'd like to add that some ESPs have their own additional restrictions on mailbox names, plus it's usually helpful to verify the existence of the domain par

Yep. I'd like to add that some ESPs have their own additional restrictions on mailbox names, plus it's usually helpful to verify the existence of the domain part (DNS) because people make mistakes all the time. We (Mailgun) have a free service which does all of these things: http://blog.mailgun.com/post/free-email-validation-api-for-web-forms/ http://blog.mailgun.com/post/free-email-validation-api-for-w...