Winmail.dat files are created by Outlook and can strip attachments from your email.

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

1. Click File in Outlook
2. Select Options
3. Go to the Mail category
4. Ensure HTML or Plain Text is selected for Compose messages in this format under Compose messages
5. Ensure 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 prevent winmail.dat attachments in Outlook 2002-2007:

1. Select Tools | Options... from the menu
2. Go to the Mail Format tab
3. Ensure either HTML or Plain Text is selected under Compose in this message format 
4. Click Internet Format
5. Ensure 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

To disable Winmail.dat from still going to particular addresses regardless of default setting

In the most recent versions of Outlook, make sure the email address is not in your Outlook Contacts

• Outlook 2016 currently does not offer a way to change sending preferences for email addresses that are assigned to an address book entry.

1. Open an email from the desired email address or start a new message to that address
2. Click on the address with the right mouse button
3. Select Outlook Properties from the menu that appears

In Outlook 2007-13

1. Search for the desired contact in your Outlook Contacts
2. Double-click the contact's email address
   • Alternatively, click on the desired email address with the right mouse button 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

Additionally, in all versions of Outlook, you can disable TNEF completely, which will allow the attachments to be sent correctly.

1. In Outlook, from the Tools menu, click Options Mail Format
2. In the "Send in this message format" list, click Plain Text or HTML
3. Click Ok