Postfix unknown command
You could use smtpd_command_filter to exclude these bogus commands:
In /etc/postfix/main.cf:
smtpd_command_filter = pcre:/etc/postfix/bogus_commands
And in /etc/postfix/bogus_commands:
/[^ ]{3}/ NOOP
You may have to test the regex to fit your needs.
There are no SMTP commands that consist of 3 letters :)