Scenario: We have an Exchange Server 2007 (itropics.net) in house. When our supplier (supplier.com) sent us an email, they receive a bounced back error message: mail.itropics.net #554 5.1.0 Sender denied
To fix that, make sure that the supplier's domain named is not being blocklisted by your Exchange Server. Go to Exchange Management Console -> Organization Configuration -> Hub Transport -> Anti-spam -> Sender Filtering and make sure that the supplier domain name is not on the list. Remove it if it's there.
Or check the recipient's (This email address is being protected from spambots. You need JavaScript enabled to view it.) Blocked Senders to see if the domain is in the list. You can do it by checking Outlook (go to Tools menu then Junk E-mail Protection) or logging into OWA