To set up an SMTP relay, you must first create a connector in Office 365 to accept SMTP transactions. Note, that you cannot use self-signed certificates or certificates issued by an internal PKI because they are not supported. *.) issued by third-party certificate authorities like LetsEncrypt, Digicert, etc. This certificate could be a wildcard certificate (eg. TCP port 25 access from a client to your Office MX endpoint.You can still create an SMTP relay in Office 365 even without this, but you might not test. A Windows 10 computer with PowerShell v5.1.If you do not have this yet, you can Sign up for an Office 365 E5 trial subscription. These are the requirements before you can proceed. Validating SMTP Relay Hops using the Message Header.Testing a non-TLS SMTP Relay (IP Address).Testing the Office 365 SMTP Relay Connector with PowerShell.Create a TLS Connector using PowerShell.Create a Non-TLS Connector using PowerShell.Create a TLS Connector using Exchange Admin Center.Create a Non-TLS Connector using the Exchange Admin Center.