When third party application or mails from the internet sends a message to your organisation, where you have Edge-Transport Server is receiving server, then You may get an NDR saying “Unacceptable Attachment”
The attachment sent by the application or by users in the internet may be listed in the AttachmentFilterEntry.
But in some cases the attachmet get rejected by Attachment Filter Agent, though its not listed in AttachmentFilterEntry
The reason being is that the attachment may not be recognised by the Attachment Filter Agent. For example “.zip” file may get rejected by the Attachment filter Agent
This is a know issue and update rollup 5 wil fix this as per Microsoft.
*But even the a different attachment other than .zip can be rejected due to the same reason
1. Install the recent rollup update
2. Add a key in the file EdgeTransport.exe.config file, under <appSettings> </appSettings>
<add key=”AllowInvalidAttachment” value=”true” />