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.