Our Exchange server was refusing mail

451 4.7.0 Temporary server error. Please try again later. PRX5

Attempts to send mail would connect, send data, and then hang for a few seconds before returning the tempfail error.

Looks like there’s “sender reputation” data stored at .\Exchange Server\V15\TransportRoles\data\SenderReputation that is used. Since I’m not actually doing filtering on the Exchange server, stopping the transport services, moving the files out of the folder, and then re-starting the services rebuilt the data and allowed mail to send again.