- Go to Postbox → Settings (or Tools → Account Settings on Windows), and select the account you want to send protected attachments from.
- Scroll to Outgoing Server (SMTP) and click Edit SMTP Server List….
- Click Add to create a new outgoing server, or select the existing one to edit it.
- Enter the gateway's inbound server settings — server address and port.
- Set Connection security to match the port (SSL/TLS for 465, STARTTLS for 587), and enter your account email as the username under Authentication.
- Save, then make sure this server is selected as the account's outgoing server back on the main Account Settings screen.
Postbox is built on the same underlying engine as Thunderbird, so if a menu here
doesn't quite match what you see, the [Thunderbird guide](/setting-up/thunderbird)
describes the same settings under very similar names.