When you email a form such as a paystub or invoice, the recipient of the file receives a file attachment called Winmail.dat instead of the .pdf file that was sent.
This is a known issue for Microsoft Outlook If the recipient's email client cannot receive messages in rich text format (.rtf).
How to fix it
To configure the e-mail client and to prevent the Winmail.dat file from being sent, refer to the Microsoft Support article How to prevent the Winmail.dat file from being sent to Internet users.
Set up your email service to have QuickBooks Desktop use another email option.
Note: If only one or some customers experience this issue, it can be due to a problem on the receiving end. Certain security and mail filters may detect these emails as mass emailing and must be adjusted accoringly.