- In Spark, go to Settings → Accounts, and select the account you want to send protected attachments from — or add it fresh as a generic IMAP/SMTP account if you haven't yet, rather than signing in through Google or Microsoft directly.
- Open that account's Server Settings and find Outgoing Mail Server.
- 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 delivery goes through the gateway.
Like the desktop version, Spark's mobile app signs Google/Microsoft accounts in
via OAuth by default, which doesn't expose a custom SMTP field — add the account as
plain IMAP instead if you don't see these settings.