The sender is receiving a bounce back with the error "550 5.7.1 Message rejected as spam by Content Filtering"
Error: "550 5.7.1 Message rejected as spam by Content Filtering"
This error is generated when the recipient mail server is using Microsoft Exchange content filtering. MS Exchange servers have built in filtering features that can block certain emails if they match their content filtering rules. Email messages that are empty in content (or nearly empty) may be caught by one of the default filtering options. This error is generated by the default-enabled Content Filtering anti-spam option under Hub Transport > Anti-Spam in the Exchange configuration.
Steps for the administrator of the recipient server to resolve the issue: