I'm facing a serious problem on a clients MacBook Pro (High Sierra). He has transferred his domain to a new provider, where I recreated his email accounts and added them in Mac Mail before the migration took place, so he had both accounts in his list. I expected to do a clean mail migration once new messages started pouring into the new account.
However, I seem unable to move the messages from the (now defunct) old account into the new one. The move progress stays at 1/316, taking over an hour now.
I also attempted to export the inbox and sent folders, which produces some empty mbox files. Reimporting them should - according to the Apple help pages - create a new imported folder in the folder list, but it does not.
Is this expected behaviour? How can I get those emails into the new account?