There is a limit of 100 emails at a time, so it shouldn't be a problem. I've tested it myself, and 9 emails go fine - but as soon as I add the 10th and send an email, the logs say 'Could not instantiate mail function'.
I'm trying to get the return path to be
$Email->ReturnPath = "".$rsUser->getColumnVal("UserEmail") ."";
But that always results in failure ('Could not instantiate mail function' in the logs)
I've tried all the variants I can think of, without success - it only works if there is an actual email address entered in the appropriate place, e.g. firstname.lastname@example.org.