Well worth noting: auto-correcting the email addresses would be a very bad idea; ditto for stopping them from submitting the form if this is triggered.

But just showing a little note "are you sure you didn't mean username@gmail.com", then letting them submit anyway, isn't generally annoying even for misfires.

Because there actually are going to be occasional users who mistype "ymail.com" when they mean gmail. Hey, thanks for the warning; the y is right near the g.