This problem occurs on the side of whomever sends the email. Below are the instructions to fix it for them.

Outlook 2010, 2013:

  1. Click File, then Options.
  2. Click ‘Mail’ on the left, then scroll down the main options window to the ‘Message format’ section near the bottom.
  3. There is a setting there labelled “When sending messages in Rich Text format to Internet recipients:” followed by a drop-down box.
  4. Select ‘Convert to Plain Text format’ or ‘Convert to HTML format’ and click the ‘OK’ button.

From now on Outlook will send messages destined for external recipients using plain text or HTML, and will not convert the messages to the special format that only Outlook understands.

 

Outlook 2003, 2007:

  1. Click Tools, then Options.
  2. Select the ‘Mail Format’ tab.
  3. At the top of the tab, in the ‘Message format’ section, use the “Compose in this message format:” drop-down box to choose either ‘HTML’ or ‘Plain Text’. Online Computers & Communications, LLC
  4. Click ‘OK’ to close the Options dialog box.Moving forward, any new emails you send to recipients that are not a part of your company

should not receive any WINMAIL.DAT attachments, and should receive emails as you composed them.

 

To fix for specific contacts only:

  1. Locate and open the contact that you want to affect.
  2. Start Outlook, go to Contacts (called ‘People’ in Outlook 2013).
  3. NOTE:If using Outlook 2013 and your People open in the ‘People’ view, change the view to any other.
  4. Find the specific contact and double-click the contact record to open it.
  5. Double-click the E-mail address of the contact record, and you’ll see an additional options box open up:
  6. Select the preferred format for this specific contact, then click ‘OK’. Outlook will use that format from now on when sending to that recipient.