This is on the latest version, just installed it last week. I've been testing a new MyBB setup on a new server. Turns out that my ISP on this new system has port 25 blocked. I noticed that any new test signups (despite having selected the email and admin confirmation option) immediately went to 'ready to confirm' state, despite the fact that no e-mail was delivered. So, apparently, if the send of the confirmation e-mail fails outright, as in can't be sent, there is no warning. The account just becomes ready to confirm. It would probably be better to issue an error and leave it in 'waiting for e-mail validation' type state. Otherwise, it would leave a hole for invalid signups to be approved if something went wrong with outgoing e-mail and the admin didn't realize it.
I definitely hope that, if the validation email just can't be delivered or something that the above doesn't happen, and it's only because of some really fundamental (and fairly pathological but still possible) issue with outgoing e-mail delivery. Otherwise, anyone could get a bogus account by just providing an invalid return e-mail. The admin wouldn't know any better and just confirm the signup.
I definitely hope that, if the validation email just can't be delivered or something that the above doesn't happen, and it's only because of some really fundamental (and fairly pathological but still possible) issue with outgoing e-mail delivery. Otherwise, anyone could get a bogus account by just providing an invalid return e-mail. The admin wouldn't know any better and just confirm the signup.