Apple Mail lets you add the gateway as an additional outgoing server on an existing account, without setting up a whole new mailbox.
- Open Mail → Settings → Accounts, and select the account you want to send protected attachments from.
- Go to the Server Settings tab and find Outgoing Mail Server (SMTP).
- Click Add Server… (or edit the existing outgoing server, if you'd rather replace it than add a second one).
- Enter the gateway's inbound server settings — server address, port, and your account email as the username — and enable Automatically manage connection settings off if you need to set the port manually.
- Set the connection security to match the port you used (STARTTLS for 587, SSL/TLS for 465).
- Close Settings and send a test message to confirm it goes through the gateway.
If your account is iCloud Mail or another provider that only supports its own
built-in outgoing server, adding a second custom SMTP server may not be available —
this works for standard IMAP/SMTP accounts (Google Workspace, Microsoft 365, most
business and ISP mailboxes).