Please confirm whether user is using mac or windows?
Attachment format changing to winmail.dat for some mails.
https://learn.microsoft.com/en-us/troubleshoot/outlook/message-body/how-tnef-affects-email-messages
https://www.slipstick.com/problems/outlook-is-sending-winmail-dat-attachments/
https://www.lifewire.com/prevent-sending-winmail-dat-attachments-1173717
Apple and mac don't support Outlook Rich Text Format (RTF) it convert attachment into .dat when it received at apple or mac client.
https://www.transip.eu/knowledgebase/entry/99-get-winmaildat-file-attachment-email/#:~:text=dat%20file%20as%20an%20attachment%20in%20my%20email%3F&text=If%20you%20receive%20an%20email,message%20also%20uses%20Microsoft%20Outlook.
Please follow below steps.
Change message format Outlook Rich Text Format (RTF) to plain text or html.
follow below link
Video link
https://www.youtube.com/watch?v=VSgbG_-8iCo&ab_channel=Gimmio