Why did I get the sendmail message: "SMTP attack: command=NOOP, count=20"?

Solution In Progress - Updated -


I got the following NOOP message in sendmail.

Nov  9 10:01:37 rhel73 sendmail[15974]: 1A90xlpf015974: localhost []: possible SMTP attack: command=NOOP, count=20

What does this mean?

How should I do for this message?


Red Hat Enterprise Linux 8
Red Hat Enterprise Linux 7

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