Wednesday, June 29, 2016

Winmail.dat attachment problem from Exchange Server or Exchange Online

By default, email messages that are sent from Exchange Online in Office 365 (same for Exchange Server) use the Transport Neutral Encapsulation Format (TNEF) format. Messaging systems that aren't based on Microsoft Exchange may be unable to interpret messages that use this rich text format (RTF). If the recipient’s messaging system can’t process this format, a file attachment that's called "Winmail.dat" is added to the message.

Office 365 admins can use Windows PowerShell to change the message format to prevent the "Winmail.dat" attachment from being sent to external recipients. 
MS suggests you (kb2487954) to configure it per external contact or per external domain. However you may find this article useless if you'd like to not fix it again and again for every new detected domain or external contact. Proposed solution is to make global change for any external domain.

1. Connect to Exchange Online PowerShell or run Exchange Management Shell: