Funny story, actually not so much – I “almost” lost all my data on my main system and a back up drive in a weird flood (water) incident and then a burnt out hard drive on the sole survivor laptop. Using a bunch of tools, hot air gun and coffee I restored about 99% of everything – but there were -some- email restoring issues.
Turns out you can’t just drop the Apple Mail.app messages (emlx) in to your current Mail.app email or import them, it’s bizarre. But you can convert the emlx mail to a mbox and then import them, all with a free tool – after that you’re back in business. There might be a more elegant way of doing this, but it worked for me…
emlx to mbox converter – Link.