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Why are some email attachments created via Salesforce for Outlook or Email to Salesforce, saved as 'winmail.dat'

Knowledge Article Number 000047811
The winmail.dat attachment behavior is a known issue with Microsoft Outlook using Exchange Server, and the issue is not related to the Salesforce inbound messaging system.  If this formatting is sent to ANY of our email services this behavior can be seen and not exclusive to Salesforce for Outlook and Email to Salesforce.

An email message that contains a winmail.dat attachment could occur when following conditions hold true: 
  • The email message is sent by someone using Microsoft Outlook.  
  • The format of the message is Rich Text format (RTF). 
Following are Microsoft KB's that explain why it happens and how to prevent it: 
Since this behavior is controlled by settings on the sender's email server and or the users computer, this is out of scope in regards to what Salesforce can do from a technical standpoint.

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