Airmail 3 is a sleek and featureful alternative to Apple Mail on MacOS. We chose this application as a target for reverse engineering to gain a better understanding of how MacOS applications work on a low level. We decided to focus our efforts on the custom airmail://://:// URL scheme that is registered by the application. The only parameter that requires prior knowledge is the account parameter. This parameter determines from which configured a new email message is sent. The compose and send commands relate to new messages, and do not require a mail or messageid.”]
Source: https://versprite.com/blog/security-research/phishing-airmail3-mac/