daily otput: rejected mail hosts?

[Anton Shterenlikht]

I see in the daily output:

Checking for rejected mail hosts:
 172 553 check_mail system.mail exist
 129 553 check_mail tsvpt014.vpt.co.uk exist
  43 553 check_mail unix.dedicated.com.tr exist
  43 553 check_mail ubs.net exist
  43 553 check_mail localhost.localdomain exist
  43 553 check_mail journal-cfp.org exist
  43 553 check_mail italiasito.it exist

What is that about?

Is this described somewhere in sendmail manuals?

Thanks

Anton