- Open Preferences → Accounts, and select the account you want to send protected attachments from.
- Go to the Sending (SMTP) section of that account's settings.
- Enter the gateway's inbound server settings — server address, port, and your account email as the username.
- Choose the security type matching your port (SSL/TLS for 465, STARTTLS for 587), and enter your mailbox password.
- Save. Mailspring checks the connection automatically when you save a changed outgoing server, so a failure shows up immediately rather than on your next send attempt.
Mailspring keeps incoming (IMAP) and outgoing (SMTP) settings separate — this only
changes where your outgoing mail is sent from, not how mail is fetched into the
app.