The error happens when SMTP server's authentication mechanism differs from the one that CA SDM is set with.
To verify if authentication is required or not, a test via telnet might be a good idea:
telnet <SMTPServer> <SMTP_PortNumber>
It would indicate there normally if authentication is needed or not. Below is a document from Microsoft that covers it in detail: https://docs.microsoft.com/en-us/exchange/mail-flow/test-smtp-with-telnet?view=exchserver-2019