Setting Up

Aqua Mail

Add the gateway as an SMTP account in Aqua Mail.

Aqua Mail is built around manual IMAP/SMTP setup, so it exposes the outgoing server directly for any account type.

Open Menu → Settings, select the account you want to send protected attachments from, and go to Outgoing server (SMTP).
Enter the gateway's inbound server settings as the Server hostname and Port.
Set Security type to match your port (SSL for 465, STARTTLS for 587).
Under Sign in, enter your account email as the username and your mailbox password.
Save, then use Aqua Mail's built-in Test settings option before sending anything for real.

Point a PHP application's outgoing mail at the service.

Point a Rails application's Action Mailer SMTP settings at the service.

Point a Python application's smtplib client at the gateway.