I gave up on Airmail as a result of this. I did find a very longwinded way to finally extract myself from Airmail's proprietary local messages store, which seemed to work out, but wasn't very user-friendly. If I recall correctly, I had an IMAP account (hotmail possibly) configured as an account, and was able to create like named folders on outlook.com, and then copy them over from my local files to outlook.com. It was a long, complicated process, but I was finally able to get out of using Airmail while preserving my email history. As much as I liked Airmail (although I was never happy with the slow way it seemed to download email messages), I wasn't willing to stick with a platform where there was not easy and obvious way to back myself out of it.
I now use the built in OS X Mail client, integrated with Outlook.com. Works well, and no regrets.
We need the ability to export our local folder email into a format that can be used to import into another email solution, if the need ever arises. I truly regret moving to Airmail as things stand, because there is no way to take my email with me if I need to migrate at any point in the future. While I don't plan on migrating off Airmail, I'll be stuck if you ever abandon the product for any reason. Please fix this issue. Last time I asked about this was over a year ago, and I'm surprised there's been no progress since then.
Agreed, this is essential. I switched from the Mail app to Airmail, and can't see any way to switch back from Airmail as a result of the lack of export capabilities. While I have no immediate plans to switch mail clients, having no viable way to make that transition in the future is a major concern. This should not be a one way process where my data is trapped.