How to resolve the winmail.dat attachment issue in Microsoft Outlook

Case

You notice that when sending emails in rich text format from one Exchange Server on-prem or Exchange Online mailbox to a specific recipient, the recipient receives a plain text email (email body) with all formatted information encoded as an attached winmail.dat file.

Both Microsoft Outlook and the Microsoft Exchange Client sometimes use a special method to package information for sending messages across the Internet. This method is technically known as Transport Neutral Encapsulation Format (TNEF). The use of TNEF is typically affected by settings in Outlook that are known as Microsoft Outlook Rich Text Format (RTF). RTF and TNEF are closely related, but not the same. A TNEF-encoded message contains a plain text version of the message, and a binary attachment that “packages” various other parts of the original message. In most cases, the binary attachment is named Winmail.dat.

Solution

Client side

If you are not interested on a permanent fix of the root cause of this issue and can live with keeping the winmail.dat attachments, you can make use of a winmail.dat file reader which will allow you to decode the attached winmail.dat files and read all content originally sent from the sender. Some examples of free winmail.dat readers are available below:

If you need to resolve the issue, then you should follow the steps below:

  • In the Outlook client options (File–> Options–Mail) set the “compose messages in this format” setting to HTML. This setting must be enabled in both the sender and the recipient email clients.
  • In same Outlook options pane as above, set the “When sending messages in Rich Text format to Internet recipients: Convert to HTML format” option. This setting must be enabled in both the sender and the recipient email clients.
  • Although not critical for this case, it would be good to check that both the sender and the recipient have the same message character set encoding option set in their email clients, for example UTF-8.
  • Try sending the same email message via Outlook Web App (OWA) instead of the Outlook desktop client by having the message format settings properly setup.

Server side

Similarly with the client side actions described above, you can carry out same email message format management operations by utilizing Exchange Server on-prem or Exchange Server Online Powershell cmdlets. The following are two viable options:

  1. Change the message format for external contacts
  2. Change the message format for all messages that are sent to a specific domain

Sources

https://support.microsoft.com/en-us/topic/how-to-specify-the-email-message-format-that-s-used-for-external-recipients-to-prevent-winmail-dat-attachments-4a379475-1557-9554-ab72-91c5867afc11

https://docs.microsoft.com/en-us/exchange/mail-flow/content-conversion/message-encoding?view=exchserver-2019

https://support.microsoft.com/en-us/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722

https://support.microsoft.com/en-us/topic/winmail-dat-sent-as-an-email-attachment-in-outlook-2007-and-2010-b72c81ac-db5e-b142-d43d-448efbc23cce

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