It might work but spammers will start using real email addresses of innocent people and we will be spamming those people with the confirmation emails and they will report us as spammers :/

Yes you correct I rarely think ahead much :)

Is it possible just to check that the address is real by some sort of ping to the email server without the recipient, if real, knowing ??

Indeed it would be possible to check if anywhere.crap has a real email server running, but in case it has not the timeout delay will be atrociously long to endure for the user (especially if he made a typo like gmai.lcom)

But if done asynchronously with a scheduled task this could be interesting.