An error occurred while saving the commentSimon 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.