If you're trying to optimize a registration process, you want to minimize the number of people you lose at each step. If someone gives you a bad email address, and then you try to send the activation link there, you've lost that registration. If you catch it when they submit the form then you've got a better shot at getting a complete registration from that user.
But there are several businesses that tried to do exactly that and lost me as a customer since they didn't accept my perfectly valid address with a + in it.
Re:Valid Email Addresses
by hamburglar on Friday November 30, 2012 @07:55PM
(#4856354)
The key is that your validation is not about "accepting addresses you know are good" but "rejecting addresses you know are bad." If I enter foo@gmail@com as my email address, it's bad user experience to make me sit around refreshing my inbox and wondering where the confirmation email is.
When damn near every application today requires the user to click a link sent to their email, I can't envision any reason for actually validating the email at form submission time (except len <= 254). If they click the activation link, it's a legitimate user. End of story.