But your way, if the user makes a typo like "foo@@bar,com" then he will expect to receive an email but wont. Better UX in my opinion is to do validation before sending, both client-side and server-side (esp. good for REST JSON APIs), then if these are all good send the welcome email.
I wouldn't mind using a simple regex or validator to check the e-mail addr for validity.
It wouldn't be RFC-compliant, but it would catch 99% of typos.
Instead of being an error when the e-mail fails validation though, it would say something like: "your e-mail does not appear valid; please double check your entry. You will be sent an activation e-mail; click [Continue] if you're sure the address is valid."
Basically if it fails the "99%" test, then if that fails, let the user decide if their e-mail is in the 1% or not.
