The driving force is that you want to correct an invalid email ASAP, preferably in the client with live feedback coloring, etc.. Most email services I know don'

The driving force is that you want to correct an invalid email ASAP, preferably in the client with live feedback coloring, etc.. Most email services I know don't give you any immediate feedback, and some only give you a basic check that can bounce later. So claiming you just have to check for an @ sign and try sending means there is going to be a huge delay before you know about the error. Saying the user will just come back and register again is not good. That's like saying if your page is very slow to load, users will just wait for it to load. They don't. They leave and never come back most of the time. If you can't afford to write the code to help the users fill out the form in a way that will work, fine, that's something you didn't have time for considering the percentage of users it will help/retain. But don't claim it is useless.