1. Login ECP Panel > Mail flow > Receive connectors > Add a New Connector
  2. Select Frontend Transport and type is Custom
  3. Security
    Authentication enables “Externally secured (for example, with IPsec)
    Permission Group enable “Exchange Servers”, “Partners” and “Anonymous users”
  4. Remote network setting:
    add which IP address allow relaying the email
  5. Under accepted domains, you can add another domain for the relay, else you only can using by default domain for the relay.
    Remark: When you using another domain for the external relay, please also check the SFP record, make sure the SPF record has included this SMTP server IP, so the outgoing e-mail will not classify as spam e-mail.