Some emails may contain the email attachment "winmail.dat". This often raises the question of what the file winmail.dat means and how it can be opened.
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 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 in this Microsoft article.
Solution for email recipients
If no Exchange server is used, the question arises how winmail.dat attachments can be opened:
- 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.
- You also have the option of using an online tool of your choice for further analysis of the file.
- Use a free tool, such as Winmail Opener for Windows
Sustainable solution for the email sender
Deactivation of the TNEF format via PowerShell command, which ensures that emails are no longer sent in the TNEF format.
Caution, this will disable this feature for the entire O365 domain.
Set-RemoteDomain -Identity Domain -TNEFEnabled $false"