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.
https://support.microsoft.com/en-gb/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722

Change message format Outlook Rich Text Format (RTF)  to plain text or html.
follow below link
https://support.microsoft.com/en-gb/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722 


Video link
https://www.youtube.com/watch?v=VSgbG_-8iCo&ab_channel=Gimmio