- In Spark, go to Settings → Accounts, and select the account you want to send protected attachments from — added as a generic IMAP/SMTP account rather than through Google or Microsoft sign-in, so the outgoing server is editable.
- Open Outgoing Mail Server under that account's server settings.
- Enter the gateway's inbound server settings — server address and port.
- Match the security type to your port (SSL for 465, STARTTLS for 587), enter your account email as the username, and save.
- Send a test message to confirm delivery goes through the gateway.
As with the iOS version, a Google or Microsoft account signed in directly through
Spark won't expose a custom outgoing server — add the account as plain IMAP
instead.