Why is there a Winmail.dat attachment in my email?

The email attachment “winmail.dat” might be present in some emails. This frequently prompts inquiries about the purpose of the winmail.dat file and how to open it.

The issue

When a user of Microsoft Outlook writes or receives a message, it frequently includes an attachment in the TNEF format called “winmail.dat” in addition to regular text. This text is frequently unintelligible to the recipient.

The reason

The issue that has been reported is related to the Microsoft Exchange Server, which affects all mail servers that are not Exchange. Rich Text Format (RTF) was the root cause of the issue. By doing this, you may be sure that email content is embedded with the “Transport Neutral Encapsulation Format (TNEF)” technique. This appends the “winmail.dat” attachment in the previously described TNEF format to the email automatically. This contains all file attachments in addition to the prepared text. This format can only be opened by Microsoft Outlook users because it was created with Outlook.

A remedy for those who receive emails

There is a question about how to open winmail.dat attachments if there is no Exchange server in use:

MDaemon Technologies’ mail and groupware server MDaemon Email Server has a built-in feature that makes it possible to open attachments with the extension winmail.dat. Open the MDaemon GUI, then select Security → Content Filters → Compression → Attachments from winmail.dat files (text messages from Microsoft Outlook in RTF format) to accomplish this. By selecting this option, the winmail.dat attachment’s actual files are guaranteed to be extracted and transformed into regular file attachments. This indicates that webmail can display them normally.
For more in-depth examination of the file, you can also use any web tool of your choosing.
Make use of a free utility like Winmail.dat Reader.

Process for Outlook users

As long as emails are sent in the “HTML” or “Plain” formats, the scenario doesn’t arise. Users of Outlook can quickly transition from RTF to HTML format, as described in this article from Microsoft’s support website.

Long-term resolution for the e-mail sender

Emails transmitted in the TNEF format are guaranteed to stop when the TNEF format is deactivated using a PowerShell operation.
Be advised that doing so would deactivate this feature for the whole O365 domain.

Set-RemoteDomain $false -Identity Domain -TNEFEnabled"
Why is there a Winmail.dat attachment in my email?

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