what are .dat files?

This will be a bit of a technical explanation to help you understand and fix this issue.

A .dat file is a generic data file which contains info about the program that created that file. For outlook, most likely the .dat stores information about mails sent in Rich text format (RTF) ) It is a common outlook error that RTF and format are converted to .dat or winmail.dat when sent or received as mail attachment. This will happen when you sent an email and the receiving party's computer does not support RTF format. The receiving party will receive the email's attachment as a winmail.dat

The same goes up when you receive an email in RTF and your Mac is not opening it with TextEdit, then you will receive a winmail.dat file.

The trick in this is to try and never sent an attachment as RTF. You can see this link from Microsoft support on how to change your message format to HTML or plain text instead

https://support.microsoft.com/en-us/office/change-the-message-format-to-html-rich-text-format-or-plain-text-338a389d-11da-47fe-b693-cf41f792fefa?ui=en-us&rs=en-us&ad=us

Hope this helps!