When Letter Opener converts a winmail.dat attachment, the Letter Opener banner appears on top of the email message.
Some winmail.dat files just contain the message text with additional rich text formatting and no attachments. They are still sent as winmail.dat file, even though the message only contains text.
This is often the case with message signatures. For example see the use of bold font for the name and phone number prefix here:
If your banner shows something other than the conversion message click the following link: