Hello, totally new to posting and Zimbra.
Our Zimbra server has been working nicely for the past year. Now we have a client that using software called Millennium to send appt reminder emails. The software uses SMTP. Currently when they try and send, the error is 554 5.7.1 : Relay access denied.

Our Zimbra has MTAAuth True and TLS is True. All other clients us the secure SSL connection, else Auth access fails.
I am including a screen grab of the Millennium set-up. If anyone can provide any insight where to look in Zimbra or what the settings on their software should be. Thank you in advance.