Why do messages contain unreadable “winmail.dat” attachments?

If you use Microsoft Outlook to send mail, people might complain that your messages contain an unreadable “winmail.dat” attachment, with the normal attachments not visible.

Or if you receive mail from people who use Outlook, the messages they send you might contain a “winmail.dat” attachment instead of the attachment you were expecting.

If this happens, it’s because the copy of Microsoft Outlook sent the message in a non-standard “rich text” format. Those attachments can only be viewed using certain mail programs like Outlook, but not by many non-Microsoft mail programs or our Webmail system — the “rich text” part appears as a “winmail.dat” attachment in those programs.

To solve this problem, the sender of the message needs to make their copy of Outlook send mail using a standard format instead of “rich text” format. Confusingly, recent versions of Outlook have three separate settings that can affect this. Here’s what to do:

First, click the File tab, then choose Options in the left column, then Mail, so you see a screen like this:

Outlook settings

Make sure that “Compose messages in this format” is set to HTML. Then scroll down that window to the “Message Format” section:

Outlook settings

And make sure that “When sending messages in Rich Text format to Internet recipients” is Convert to HTML format. Then click OK to save those settings.

Finally, just before you send a message, click the Format Text tab and make sure the format is set to HTML:

Outlook settings

Be sure to do this as the final step before sending the message, because this setting can change by itself when you add recipients.

If you still have trouble, try these same steps using Plain Text instead of “HTML” in all three places to see if that fixes it. If it does, switching it back to HTML again after doing that test may make it work, but if not, you’d need to contact Microsoft for help with that bug (it should be impossible for Outlook to send “winmail.dat” attachments in that situation, and if it’s doing it anyway, there’s something wrong with it).

Other resources

One final tip we’ve seen online but haven’t been able to verify ourselves is that if the problem is only happening when you send messages to certain people in your address book, but not to everyone, you may be able to fix it by deleting that address book entry and re-adding it (see the February 8, 2017 reply from “Rafiq Hirji” in this thread on the Microsoft forums).

This Microsoft page has more information about all of this, and you may also want to check out this third-party page that explains how to solve the problem.