You sent an email to your client with an important attachment from your Outlook and think it will go through as it always does. But instead, your client replied back saying that the attachment is an unknown forward because it’s called windmail.dat.
What to do?
According to Microsoft, the reason why the normal attachment ends up as windmail.data at the other end is because the email was composed as Rich Text message Format (RTF) and therefore the email program on other platforms like Mac OS, iOS, or Android cannot interpret correctly.
The resolution is simple, just send e-mail in either HTML or plain text format.
Go to File > Options in Outlook, and make sure HTML is selected as the default composing format in Mail tab.
You can also set the message format to HTML in Outlook message window as well. Just go to Format Text tab and make sure As HTML or As Plain Text is selected.
But what if changing to HTML format still didn’t solve the problem?
That’s probably because the Rich Text Format was set by default to the specific email address. To address that, all you need to do is to clear up the email address cached in your Outlook.
Open a new message window, type the email address you were having the issue with. When the address pops up in a drop-down list, click the X icon next to the address to delete it from Outlook cache.
What if I am the one receiving the windmail.data email?
Rather than asking the sender to resend the email with the proper format, you can use a free tool called Winmail.data Reader to convert the windmail file to a proper Outlook message file.