Email attachments are showing as winmail.dat files

There is a long-standing "problem" between Outlook and Internet email - Outlook uses a proprietary email format that only Outlook and Exchange (and a few other email programs, including Gmail) can decipher. When the recipient doesn't use Outlook, they get a plain text message and an attachment named winmail.dat. Additionally, the attachment icon may be hidden by some clients, including OE and Windows Mail, making it look like the attachment was removed.

The problem? Any attachments included in the message are encased in the winmail.dat attachment.

How do I fix it?  

Prevent Winmail.dat attachments from being sent in Outlook

1. Click File in Outlook.
2. Select Options.
3. Go to the Mail category.
4. Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
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.
6. Click OK.

To make sure Outlook 2002 to Outlook 2007 do not attach winmail.dat files:

1. Select Tools | Options... from the menu.
2. Go to the Mail Format tab.
3. Under Compose in this message format:, make sure either HTML or Plain Text is selected.
4. Click Internet Format.
5. Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format:
6. Click OK.
7. Click OK again.

In Outlook 2016:

1. Make sure the email address is not in your Outlook Contacts.
2. Outlook 2016 currently offers no way to change sending preferences for email addresses that are assigned to an address book entry.
3. Open an email from the desired email address or start a new message to it.
4. Click on the address with the right mouse button.
5. Select Outlook Properties from the menu that appears.

​In Outlook 2007 to Outlook 2013:

1. Search for the desired contact in your Outlook Contacts.
2. Double-click the contact's email address.
3. Alternatively, click on the desired email address with the right mouse button and select Open Outlook Properties or Outlook Properties from the menu.
4. Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format.
5. Click OK.