My best guess is that the mail transport agent on the server is rejecting the mail because something is different from mail that is send through other places in your django code. Perhaps the mail "sender" is not correct, for example?
Unfortunately, looking at the mail log is the best way to see what's happening. If that's not available, you might want to see what is being sent.
Set this:
EMAIL_BACKEND = 'django.core.mail.backends.console.EmailBackend'
and then run your project with "runserver". The mail that it tries to send will be sent to the console instead. Check out the "from" headers and make sure they make sense. Compare that to the other kinds of email that are sent.
(Make sure you're still running with DEBUG=False)