I've tried it on two sites I've built, and they are both running the same code. One works, the other doesn't. The only difference is that the one that doesn't work is on a subdomain, so the domain the email is sent from doesn't exactly match the domain the site is on. I tried sending the mail from an address with the subdomain instead, but that didn't help.
Will investigate further :)
Hope others (maybe Apple) could shed some light on the requirements?