We are having issues sending external emails to a particular domain. When attempting to send from our Exchange Server 2003 server to any user in a particular domain, we are getting bounce messages with the following error:
Received <<< 553 mailbox email@example.com is restricted (Mode: normal)
All other domains work fine on our end. After speaking with IT on the recipient's end, they are claiming this is caused by something on our end as they aren't having difficulties with receiving messages from any other domain. I'm not sure if it is something on my end or theirs - any assistance is appreciated.