I have just updated to a new 16" Mac running Catalina and used Migration Assistant to transfer all my data including Mac Mail from my previous High Sierra machine.
One of the email addresses worked after transfer and the other didn't and NONE of the emails (including mailboxes) for that address copied across. I was forced to use IMAP to download 8 years worth of mail to the new computer but none are in their correct mailboxes. I have 59,000+ messages to resort!!!
How can I transfer the Mail from my old machine without having to delete everything or somehow use Migration assistant to do it again?