Have an exception added in the Exchange server for the account and IP of the printer to whitelist it from being checked by the spam filter.
Look at it from the perspective of the Server.
Someone is sending an email to the same person in quick succession where the subject line, message body, attachment type, file name, footer etc etc are identical, the only thing different from the servers perspective is the size of the attachment.
This is not an issue for a user who sends an email occasionally , but it very much is when sending 3 or more in quick succession (under 5 minutes typically)