I keep running into a problem where I get the error "No recipient forward path has been supplied" when a cron job tries to send emails. It's been a huge pain to figure out, and I finally tracked it down to a specific user and when I var_dumped the emails involved I found the following:

string ' user@example.com' (length=17)

I trimed the space out and it started working! I'll never get that 2 hours of my life back. :-)