It’s an old problem that keeps coming up – an email arrives with a winmail.dat attachment. We’ll explain and how to fix it from receivers and senders point of view.
Winmail.dat problems were once common. They don’t happen a lot these days because Microsoft Outlook and Exchange Server both have default options to stop it. But Office-Watch.com has had two pleas for help in the last few days, so it’s still possible.
Receiver
If you’ve received an email message with a winmail.dat attachment check our web page Winmail.dat – what to do if you get one
Sending
If someone is complaining that you are sending messages with a winmail.dat attachment check this page How to stop sending winmail.dat
What is winmail.dat?
It’s Microsoft’s own method of sending emails with formatting (bold, fonts etc) in what’s called RTF (Rich Text Format) in Outlook.
There was a time when Microsoft hoped and pushed for RTF to be the main internet email format. It was the default for Outlook and Exchange Server. RTF has Microsoft owned technologies that it could license to other companies who wanted RTF compatibility.
The rise of HTML email
The market had other ideas and the open-source HTML system (already used for web pages) became the de-facto standard for email. All email programs and web email support HTML formatted emails (there are variations in how the same email might appear on different programs/devices, but they are readable).
If you send an HTML formatted email, anyone should be able to read it whether they are in a web browser, email program like Outlook, a tablet or smartphone (Apple, Android or Windows).
For most people, HTML format email is the best option with the widest compatibility for readers of your messages.
Plain text emails have a place too.
Don’t bother with the Rich Text format unless you have a specific reason.
RTF / Winmail.dat still hangs around
But Microsoft still hangs onto their Rich Text format. It has some advantages for ‘in-house’ communication within staff of the same organization (i.e. all using Outlook and Exchange Server).
So the RTF format is still available in Outlook as one of the three formatting options for an email.
- HTML
- Plain Text
- Rich Text
Outlook will reply or forward messages in the same format as the incoming message, unless you change it from the Format Text ribbon as shown above.
New messages are usually made in HTML format (the default in Outlook these days. That default can be changed at Options | Mail | Compose messages in this format.
Even if you accidentally send an RTF message to an Internet recipient, there’s an option to automatically convert the message to HTML format. It’s down at Options | Mail | Message Format | When send messages to Rich Text format to Internet recipients. The default is ‘Convert to HTML format’.
This format change happens in the background with no mark left behind. The ‘Sent Items’ copy of the message is left in RTF format and there’s nowhere we can see in Outlook that tells you of the format conversion on sending.
Even if you disable RTF conversion to HTML format in Outlook, your email may still go to out in HTML format. That’s because Exchange Server setups (including those hosted by Microsoft’s Office 365) have a setting to convert RTF to HTML for ‘external’ or non-MAPI recipients.
The receiver can tell that the message was converted from RTF to HTML by looking at the raw message code and noting this line in the HTML version of the message
<!-- converted from rtf -->
For example
