The Mail app on my Mac OSx High Sierra closes unexpectedly. How do I remedy this?
Try to reset the application state by removing the following folder:
/Users/nope/Library/Containers/com.apple.mail/Data/Library/Saved Application State/com.apple.mail.savedState
Otherwise you can try to delete the whole Mail Library contents:
Remove your Mail accounts (one by one if needed) and then verify that Mail doesn't close unexpectedly. Add them back in one by one until you determine which account has content that causes mail to crash.
You can also look into the crash logs to see if they are the same or pay attention to see if you can make Mail crash (especially when you are not connected to the internet). That would point to a specific action or specific piece of mail that triggers a bug.
Updating Mail and sending the crash reports to Apple is also a good way to get bug fixes and be sure your bug is looked at by Apple engineering.
I seems similar problem with my case. The crash report was:
Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x0000000000000000, 0x0000000000000000 Exception Note: EXC_CORPSE_NOTIFY
If your crash type is same to me, Then deleting all your accounts and re-adding them will solve your problem.