Currently Being ModeratedApr 29, 2013 7:14 AM (in response to Chuckwheel)
I have the problem ( ... cannot be dleivered) since about 1 week on and off, depending on the recipient's address. The cause seems to be that Apple mail servers are blacklisted on some spam filter sites which ISPs use. The blacklistings on such sites change frequently. I would expect Apple to ensure their mail servers are not blacklisted. Whenever I received a postmaster delivery failure notification, I forwarded it "As attachment" to firstname.lastname@example.org and email@example.com.