Did you see any confirmation at all that the email is sending? In XF 1.5 the process for sending mail is a manual deferred task, which means when triggered it should start immediately and display a "Rebuild caches" screen and display its progress.
If that didn't happen at all, then it's possible it wasn't queued at all.
Another thing to check is if on the Admin CP home page there is a message that states "There are manual rebuild processes that have been stopped before completion. Click here to complete them.". If there is, this might be your queued email job and it won't continue until you click on it.
If that isn't there and there's nothing in your Server Error Log you may want to quickly check the xf_deferred table in the database.
You'd be looking for a row that has an execute_class of UserEmail.
If none of these things seem to be there, then it would suggest it was never queued at all. That would be odd, though., if you believe you definitely went through the process to send an email.