Everything looks normal because both sides understand TNEF.
The recipient's email client (e.g., Gmail) sees the TNEF data as a winmail.dat attachment it cannot open. How to Open a Winmail.dat File winmail.dat
A file is a container created by Microsoft Outlook and Microsoft Exchange. It uses a proprietary format called Transport Neutral Encapsulation Format (TNEF) to package Rich Text formatting (RTF), such as bolding, colors, and specialized fonts, as well as meeting requests or actual file attachments. Everything looks normal because both sides understand TNEF
If you’ve ever received an email with a mysterious attachment named that you couldn’t open, you aren’t alone. This common tech nuisance is a byproduct of how Microsoft Outlook handles specific email formatting. What is a Winmail.dat File? It uses a proprietary format called Transport Neutral
When an Outlook user sends an email using Rich Text, Outlook "wraps" the formatting and attachments into this TNEF-encoded file. If you use a non-Microsoft email client—like —your software may not know how to decode it. Instead of seeing a beautifully formatted email or a PDF, you see the unreadable winmail.dat . Why Does It Happen?
The Ultimate Guide to Winmail.dat: What It Is and How to Fix It