Exchange 2010 - email attachments from an external sender delivered as .bin files

Environment:

Sender’s system:
- Exchange 2007
- Outlook 2010

Recipients’ system:
- Exchange 2010
- Outlook 2010

The issue:

A sender sends a message with an attachment (.docx, .xlsx, .pdf etc) from one system (Exchange 2007/Outlook 2010) to several recipients in a different system (Exchange 2010/Outlook 2010). Some recipients receive the attachment in its original form, but some receive it as 2 .bin files. E.g. an MS Word .docx file of 6 MB is sent and several recipients received it as .docx, but others received it as 2 .bin files:

- ATT00001.bin (6 MB)
- ATT00002.bin (4 KB)

The files can be saved locally and renamed into correct extensions, if known, and then the files open properly. In the case above, the first file was an MS Word .docx file, and the second was the sender’s signature, a .jpg image.

After some investigation, asked the sender’s admins to check the Exchange rich-text format settings at the server side. Once they changed it to “Never use”, the issue was resolved.

Organisation Configuration > Hub Transport > Remote Domains > right click the object Default > click on Properties

Make sure that “Never use” is selected under Exchange rich-text format:

I have seen the issue before, but the attachments were either converted into .dat files or completely missing when accessed from Outlook, but I have never seen attachments being converted into .bin files. It manifests in an inconsistent way, that is not every recipient is affected, which adds to the confusion.