If you really want to do checking of email addresses right on the signup page, include a confirmation field so they have to type it twice.
No. This puts the burden of checking email validity on every user, even perfectly capable valid users. If you're validating for edge cases (mistakes or otherwise invalid addresses), treat it as an edge case and don't annoy users who can type.
> This puts the burden of checking email validity on every user, even perfectly capable valid users.
? Whenever I hit a form which wants me to retype my address, I just triple-click to select the entire address, then middle-click to paste it into the confirmation field.
No. This puts the burden of checking email validity on every user, even perfectly capable valid users. If you're validating for edge cases (mistakes or otherwise invalid addresses), treat it as an edge case and don't annoy users who can type.