i have exchange server 2010 serving my company domain; example.com
on top of that we have sub domain apple.example.com, email service hosted by external party.
right now i have issue when firstname.lastname@example.org sending email to email@example.com with error "Unable to relay for firstname.lastname@example.org
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery."
DNS forward lookup zone created for example.com contain www and mail.
we also have apple.example.com zone for www.