Import mboxes outside of Apple Mail system mboxes
It took me forever to figure this out, but mbox from Apple Mail are not imported using the steps in the Knowledge Base Support page UNLESS the mbox is in the [user]>Library>Mail>V2 folder.
An mbox that is exported from Mail onto the desktop or anywhere else does not import.
Please allow an mbox that is outside Library>Mail to be imported. This is especially important for anyone who has backed up their Apple Mail mboxes.
Also, until you add this feature, please change the Support Know Base page to make this clearer.

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Simon Catesby commented
I have to second this. I just spent ages trying to figure out what was happening.
I exported from Mail then Airmail wouldn't import those mailboxes. However, it didn't give me any message as to why nothing happened. I was even following instructions I found from yourselves, admittedly for an earlier version of Airmail, which showed you how to export from Mail to do this.
Please make this a lot clearer, or ideally allow people to import from outside the Apple Mail library.