The built-in Mail app on iPhone and iPad lets you add a second outgoing server to an existing account without removing your original setup.
- Open Settings → Mail → Accounts, and tap the account you want to send protected attachments from.
- Tap the account name again to open its advanced settings, then SMTP under Outgoing Mail Server.
- Tap Add Server, and enter the gateway's inbound server settings as the hostname and your account email as the username.
- Set Use SSL on, and set the server port to match — 465 for implicit SSL, or 587 if your setup uses STARTTLS instead.
- Save, then send yourself a test message to confirm it goes through the gateway.
This works for standard IMAP accounts (Google Workspace, Microsoft 365, most
business mailboxes). An iCloud Mail account only supports Apple's own outgoing
server and won't offer this option.