Failure to send mail with external SMTP server smtp-server: 554 Service unavailable

The RHEL 8 mailx command fails to send mail.

Here is what /root/.mailrc says:

set smtp=smtp://<External SMTP server IP address>:25
set from = [email protected]

I'm using port 25, so
smtp-auth-user
smtp-auth-password
Am I correct in understanding that these statements are unnecessary?

In addition, the following error message is displayed when sending an email:

 smtp-server: 554 Service unavailable
"/root/dead.letter" 22/941
. ..message not sent.

I looked up the error message online, but I'm at a loss because there are few articles that mention specific solutions.

My personal assumptions include:

This is a small amount of information, but what do you think?

Thank you for your cooperation.