Airmail for Mac registers and uses the airmail:// URL scheme. The “send” command in the URL scheme allows an external application to send arbitrary emails from an active account without authentication.
The handler has no restriction on who can use its functionality. The handler can be invoked using any method that invokes the URL handler such as a hyperlink in an email. The user is not prompted when the handler processes the “send” command which automatically sends an attacker crafted email from the target account.