You can Download the latest Beta as test app, is full functional and often include the most advanced fixes and features.
Please note - The trial app is the same as the macOS App Store production version which you can purchase from the App Store and has all of its feature. The only differences are - It will expire after 4 weeks time AND it is a completely separate app from the production version and runs on a separate database - so when you decide to purchase the app - you will have to set it up again.
Testflight version for iOS
You can also apply for the Testflight version for iOS through this link here
If you have any confusions/difficulties while setting the production version on your Mac - please do contact support and we will guide you step by step to set it up and make your experience with Airmail app a good one :)