3139110 - No email notification received from applications based on HANA XS classic

After following Help Guide Maintaining SMTP Server Configurations to configure SMTP server in HANA XS, and setup email notification in your XS application.
There's still no email received.

By enabling XSMail debug level trace, you found below information in HANA traces:

[000000]{-1}[-1/-1] yyyy-mm-dd HH:MM:SS i XSMail           SMTPAuthentication.cpp(00074) : No authentication!
[000000]{-1}[-1/-1] yyyy-mm-dd HH:MM:SS d XSMail           SMTPConnectionHelper.cpp(00021) : C: MAIL FROM:<sender@email.address>
[000000]{-1}[-1/-1] yyyy-mm-dd HH:MM:SS d XSMail           SMTPConnectionHelper.cpp(00085) : S: 250 2.1.0 sender@email.address....Sender OK
[000000]{-1}[-1/-1] yyyy-mm-dd HH:MM:SS d XSMail           SMTPConnectionHelper.cpp(00021) : C: RCPT TO:<receiver@email.address>
[000000]{-1}[-1/-1] yyyy-mm-dd HH:MM:SS d XSMail           SMTPConnectionHelper.cpp(00085) : S: 550 5.7.1 Unable to relay for receiver@email.address
[000000]{-1}[-1/-1] yyyy-mm-dd HH:MM:SS e XSMail           SMTPConnectionHelper.cpp(00042) : Unexpected reply: 550 5.7.1 Unable to relay for receiver@email.address

• HANA XS Classic
• HANA Platform Edition 1.0
• HANA Platform Edition 2.0

Email notification, email, HANA XS, SMTP, 550 5.7.1 Unable to relay, Unexpected reply, No authentication , KBA , HAN-AS-XS , SAP HANA Extended Application Services , BC-SRV-COM , Communication Services: Mail, Fax, SMS, Telephony , Problem