Emails with the attachment winmail.dat

Some emails may contain the email attachment "winmail.dat". This often raises the question what the file winmail.dat means and how it can be opened.


The problem

As soon as a Microsoft Outlook user sends a message and receives it, the email often also contains the attachment of "winmail.dat" in TNEF format, in addition to normal text, which, however, often cannot be interpreted by the recipient.


The cause

The malfunction described concerns a problem from the Microsoft Exchange Server and therefore all non-Exchange mail servers. The error was caused by the Rich Text Format (RTF). This ensures that the information in an email is embedded using the "Transport Neutral Encapsulation Format (TNEF)" method. This automatically adds the "winmail.dat" attachment to the email in the TNEF format mentioned above. In addition to the formatted text, this also includes all file attachments. Since this format is Microsoft Outlook - made, it cannot be opened by non-Outlook users.


Procedure for Outlook users

The situation does not occur as long as emails are sent in the "HTML" or "Plain" format. Outlook users can easily switch from RTF to HTML format, as can be seen in the Microsoft help.


Solution for email recipients

If no Exchange server is used, the question arises how winmail.dat attachments can be opened:

  1. The mail and groupware server MDaemon Email Server from MDaemon Technologies offers its own function that enables the opening of winmail.dat attachments. To do this, open the MDaemon GUI and navigate to Security → Content Filters → Compression → Attachments from winmail.dat files (Microsoft Outlook messages in RTF format). This option ensures that the actual files are extracted from the winmail.dat attachment and converted into standard file attachments. This means that they can be displayed normally in webmail.
  2. You also have the option of using an online tool of your choice for further analysis of the file.
  3. Use a free tool, such as Winmail Opener for Windows