Content Filtering has been enabled on the recipients’ mail server. This configuration is incompatible with GFI MailEssentials when installed as a relay server, all email filtering should happen on the relay server. For more information on Content Filtering and how to disable it, please see the following:
If the mail server is Microsoft Exchange 2013 and 2016 server:
If the mail server is Microsoft Exchange 2010 server:
If the mail server is Microsoft Exchange 2007 server: