Tom Nelson at Lifewire writes:
Moving your Apple Mail to a new Mac, or to a new, clean install of the operating system (macOS), may seem like a difficult task, but it actually only requires saving three items and moving them to the new destination.
And if you have all your email account passwords handy, the number of files to move drops from three to two. Tom goes into great detail, which is good, but one might have to exercise some care and thought for specific cases.
Check It Out: How to Move Your Apple Mail Files From Old to New Mac