How to Prevent Winmail.dat Attachments from Being Sent in Outlook

How to avoid that annoying Winmail.dat attachment

An illustration showing winmail.dat files attached to an email.

 ©Lifewire

Do recipients of your emails complain about a mysterious attachment called winmail.dat which they cannot open, no matter what they try? Do files you attach disappear in that winmail.dat abyss? Does winmail.dat show up for some but not all recipients of your messages?

Instructions in this article apply to Outlook 2016, 2013, 2010, 2007, and 2003.

When, How, and Why Winmail.dat is Created

If Outlook sends a message using the rarely used RTF format for bold text and other text enhancements, it includes the formatting commands in the winmail.dat file. Receiving email clients that do not understand the code therein display it as a stale attachment. To make matters worse, Outlook will also commonly pack other, regular file attachments in the winmail.dat file.

Fortunately, you can get rid of winmail.dat altogether by making sure Outlook does not even try to send mail using RTF.

Prevent Winmail.dat Attachments from Being Sent in Outlook

To prevent Outlook from attaching winmail.dat when you send an email:

  1. Select File in Outlook.

  2. Select Options.

    Options button in Outlook File menu
  3. Go to the Mail.

    Mail category in Outlook options
  4. Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.

    Compose message format: HTML, Rich Text, Plain Text in Outlook
  5. Now make sure Convert to HTML format or Convert to Plain Text format is selected for When sending messages in Rich Text format to Internet recipients under Message format.

    Convert to HTML, Plain Text, or Rich Text format menu in Outlook
  6. Click OK.

Prevent Winmail.dat Attachments in Outlook 2007-2003

To make sure Outlook 2007 to Outlook 2003 do not attach winmail.dat files:

  1. Select Tools > Options from the menu.

  2. Go to Mail Format.

  3. Under Compose in this message format make sure either HTML or Plain Text is selected.

  4. Click OK.

Disable Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default

The standard settings for outgoing mail formats in Outlook can be overridden per email address. So, on a per case basis—when somebody complains about an inexplicable "Winmail.dat" attachment after you have made all the right settings changes — you may have to reset the format for individual addresses:

In Outlook 2016:

  1. Make sure the email address is not in your Outlook Contacts.

    Outlook 2016 currently offers no way to change sending preferences for email addresses that are assigned to an address book entry.

  2. Open an email from the desired email address or start a new message to it.

  3. Right-click on the address.

  4. Select Outlook Properties from the menu that appears.

  5. Under Internet format, select Send Plain text only.

  6. Select OK.

In Outlook 2013-2007:

  1. Search for the desired contact in your Outlook Contacts.

  2. Double-click the contact's email address.

    Alternatively, right-click on the desired email address and select Open Outlook Properties or Outlook Properties from the menu.

  3. Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format.

  4. Click OK.

Extract Files from Winmail.dat without Outlook

If you receive winmail.dat attachments with embedded files, you can extract them using a winmail.dat decoder on Windows or OS X.