This is from their online chat support: "You can export messages from Airmail but only in the form of one EML file per message. To export a complete folder, you can use the Rules. Please refer to this link to create rules on Airmail. http://docs.airmailapp.com/airmail-for-mac/rules-in-airmail-airmail-for-macos Please note that the export rule can never be set to run automatically like other rules. You will have to 'Apply to Previous Messages' each time you want the rule to run." Eml files can be imported to TB by drag and drop or with [https://addons.mozilla.org/en-us/thunderbird/addon/importexporttools/ ImportExportTools]. See the section 'Windows Mail (aka Windows Live Mail) - Import to Thunderbird' section [http://kb.mozillazine.org/Importing_and_exporting_your_mail here]. If your accounts are IMAP, set them up in TB and the existing folders will download. The method above is needed for local folders.