Hi there, cm-nzl!
Thanks for reaching out to the Apple Support Communities! It sounds like you have an issue with your Mac crashing when you use an external monitor and have the Mail app on the screen. Since the issue is not happening on the main monitor, we recommend taking a look at this article with steps to follow for How to use safe mode on your Mac. This will perform checks and stop certain software from starting up automatically. Once you are in safe mode, test to see if you have the same issue.
We also suggest follow thing these steps to Set up users, guests, and groups on Mac and create a test user to see if the issue is user specific or system wide.
Let us know the results and have a great day!