Killed my old MacBook in November last year, and was able to access everything I needed for the new MacBook Pro using Time Machine, apart from emails. This Time Machine drive now has backups for the new MacBook Pro, so when I open Time Machine from within Mail it only shows me the emails from the MacBook Pro, and not the old MacBook.
Any suggestions as to how I can find the old emails?
I would try importing the old emails goto mails file menu-> import mailboxes navigate to to the backup device, locate the old backup folder and navigate to users/{ussername}/library/mail/v2/mailboxes and import the mail
https://discussions.apple.com/thread/4742683 this discussion explains why you have been unable to import your old Mac mails to the new one. for the time being, you need to pick up a good apple mail backup tool that might help you import your old mails. I can suggest you Mail Backup X. It works on systems with 10.10 and later versions only. you are already having mails from an older version in your drive. Install this tool. configure your mac mail to it. now import your mails to it. export your mails to apple mail archive.
its done. source: http://www.mailbackupx.com/how-to-scenerios/how-to-backup-apple-mac-mail-mails-on-mac.html
