You'll hurt your email reputation if you send too many emails that bounce. It's worth checking everything you can before firing off an email. This includes using a decent regex and doing a lookup on the domain to make sure they have an MX record. While technically you can have a mail server with no MX record (it falls back to sending to the A record), you won't find too many mail servers configured that way in the wild. In many cases such as email marketing, protecting your email reputation is far more important than handling the 1 in a million user with an unusual email address or mail server configuration.
Also, instead of just failing to allow the email address, you could warn the user that what they entered doesn't appear to be valid, and that they should double check the address.
