Thanks for reaching out to the Community, RJ4242.
 

When an email recipient receives an attachment called Winmail.dat instead of a .pdf file for their invoice, this is a known issue for Microsoft outlook if the recipient's email client isn't capable of receiving rich text format.
 

If only one or some customers are experiencing this, it can be due to a problem on the receiving end. Certain security and mail filters may detect these emails as mass emailing. This can be fixed by adjusting your message format accordingly in Outlook. Be sure to choose HTML or plain text to prevent Outlook from attaching a Winmail.dat file when sending the email. You can find detailed steps for changing these settings on Microsoft's website.
 

Your other option is to use an email service other than Outlook when sending emails from QuickBooks.
 

Please don't hesitate to send a reply if there's any additional questions. Have a lovely day!