- In Canary Mail, go to Settings → Accounts, and select the account you want to send protected attachments from — added as a custom IMAP account rather than through Google or Microsoft's own sign-in, so its outgoing server can be edited.
- Open Outgoing Mail Server (SMTP) for that account.
- Enter the gateway's inbound server settings — server address and port.
- Set the security type to match your port (SSL for 465, STARTTLS for 587), enter your account email as the username, and save.
- Send a test message to confirm it routes through the gateway.
As with most modern mail apps, a Google or Microsoft account added through
Canary's own sign-in button uses OAuth and won't expose a custom SMTP server — add
the account as plain IMAP instead if these fields are missing.