The sender is receiving a bounce back with below error message. Receiving server is Exchange 2007/2010. "550 5.7.1 Message rejected as spam by Content Filtering"
Email messages that are very nearly empty in content or due to some other reason may be caught by one of the default filtering options turned on in Exchange 2007. This error is generated by the default-enabled Content Filtering anti-spam option under Hub Transport | Anti-Spam.
From the Exchange program group.
Open the Exchange Management Console. Next, depending on the server type, select either or:
Microsoft Exchange | Edge Transport | Anti-Spam.
Microsoft Exchange | Organization Configuration | Hub Transport | Anti-Spam.
All of the filter types are enabled by default, and if the server's mail traffic is filtered by SonicWall Email Security, it is not necessary to keep them enabled.
Open the Content Filtering anti-spam filtering type and go to the Action tab.
Then uncheck the box next to Reject messages that have SCL rating greater or equal to.
You can alternatively disable the content filtering on mail server since email security will be doing the filtering.