Updated Apple Mail, now hung and cannot send or receive emails
You can go directly to the folder where Mail saves its data through the Finder and duplicate it to store a copy elsewhere.
Inside the Finder, menu Go > Go to Folder and then paste this path:

~/Library/Mail/V5


It is that V5 folder you'd want to back up before reseting Mail.
There is also some software solutions like mailbackupx but I never used it so I can't say what it's worth.