Okay, this may be a solution for some of you as it was for me after pulling my hair out trying to figure it out. My problem was the windows 10 mail app would always crash about 1-3 sec after open. It was working fine until one strange day. I have tried everything (removed it with powershell, reset the app, reinstall from the windows store, restart, .....), yet nothing worked for me.
The problem for me was that an email that I had just received in one of my mail accounts (gmail in my case) had a calendar update attached to it and it was causing the mail app to crash. I logged into my gmail from a browser, delete that email with the calendar update, and my mail app stopped crashing.
If you know which email in which account is causing this problem, you can just use the web to log into that account and delete that email. If not you can start by deleting each account in your mail app and testing it out to see if it still crashes. Because the nature of the problem (crashes upon opening) you cannot just go into its settings and start deleting mail accounts. If you use powershell to uninstall the mail app, after you reinstall it from the windows 10 store, it will reattach your previous profiles with all your accounts. (unless someone knows how to clear it)
This method is used to keep the mail app from crashing so we can access all the mail accounts to delete it.
1. Turn off email privacy settings. (left click on START, SETTINGS, PRIVACY, EMAIL, turn OFF "Let apps access and send email".
2. Open the mail app and click on accounts (left side). (it shouldn't be functional but still opens without crashing)
4. All your mail account will show up on the right side, chose any one (or all) and start deleting.
5. Go back to the other window and turn "Let apps access and send email" back to ON.
6. Open your mail app and see if it still crashes (it may take up to 10 sec for the mail app to work after turning email privacy back on, during this period, you mail app will give you a message like "We couldn't access your email. Please check your email privacy settings .....)
7. If this solved your problem, great. If not you must of deleted the wrong email account, repeat steps 1-7 until you have deleted the correct email account that's causing the problem.
Once again, I'm not sure if this is the solution for everyone, but it is for my case and hopefully someone else can make use of it.