Prevent Outlook from sending attachments as Winmail.dat files

To ensure that Outlook does not send PDF files (or other attachments) in a winmail.dat file format, you'll want to adjust the email formatting settings to use either "Plain Text" or "HTML" instead of "Rich Text". Here's how you can change these settings in Outlook:

For Outlook on Windows

  1. Open Outlook and click on File in the top menu.
  2. Select Options to open the Outlook Options dialog.
  3. In the left pane, click on Mail.
  4. Under the Compose messages section, look for the option Compose messages in this format: and change it from Rich Text(preferred) to either HTML or Plain Text.
  5. Scroll down to the Message format section.
  6. Ensure that the option When sending messages in Rich Text format to Internet recipients: is set to Convert to HTML format(preferred) or Convert to Plain Text format.
  7. Click OK to save your changes.

For Outlook on macOS

  1. Open Outlook and go to the Outlook menu.
  2. Select Preferences.
  3. Under Email, click on Composing.
  4. Make sure that the Compose messages in HTML by default is checked.
  5. There is also an option to set the default message format when sending new emails, replies, or forwards. Make sure it is set to either HTML or Plain Text.

For Outlook on the Web

  1. Log in to your Outlook on the web.
  2. Click on the Gear icon to open Settings.
  3. Search for Mail settings, usually under View all Outlook settings.
  4. Go to Mail > Compose and reply.
  5. In the Message format section, ensure that the option Compose messages in HTML format is selected.
  6. Save your settings if necessary.

After making these adjustments, Outlook should send emails with PDF files attached directly, rather than enclosing them in a winmail.dat file. This will make sure that the recipients are able to open and view the PDF files without any issues.