What Is A Winmail.dat File?
Microsoft Outlook automatically generates a winmail.dat file when an Outlook email user composes and sends a message using the Rich Text Format or HTML Format. These formats attach the .dat file to the the end of the email message.
A winmail.dat file contains email formatting information, in an unreadable form for humans. Outlook uses this data on the receiving end to display email messages correctly.
Unfortunately, Outlook is the only email client program that utilizes winmail.dat information. This means anyone not using Outlook, who receives and opens an email in the HTML or Rich Text format, will find a winmail.dat file attached as well.
What Causes Winmail.dat Files?
Microsoft Outlook has many options, one of which is the ability to send emails using different text formats. As useful as these extra formats might be, this is the ultimate cause of winmail.dat files which might require additional programs to open.
Winmail.dat files are created and can cause problems for users of webmail and email clients such as Eudora, Gmail, Yahoo, etc.
How To Fix Winmail.dat File Problems
If you know your regular email recipients do not use Outlook, or to eliminate winmail.dat problems for your recipients completely, there are a couple of options within your Outlook program.
Option A: Every time you send an email message within Outlook. Compose a new email message as per normal. Before sending it, select Format > Plain Text from the menu bar, then send your message.
Option B: Change your DEFAULT sending mode in Outlook, thereby sending all email messages as plain text.
Outlook 2007 & older: Select Tools > Options > Email Format > Internet Options.
Select Plain Text from the drop down menu. Press OK.
Outlook 2010 and 2013: Click File > Options > Mail. Scroll towards the bottom of the dialog box.
Under the Message Format section, where it says; “When sending messages in Rich Text format to Internet recipients:” select Plain Text from the drop down menu.
Both Options A and B will prevent winmail.dat problems for non Outlook email recipients. It’s completely up to you as a user which option you select.