In my daily report:

The following n subscribers are currently being monitored:

Err First Last  Address
--- ----- ----- -------
  2 08/13 08/13 [log in to unmask]
                Last error: Mailer whosis3.whatcha.NET said: "501
5.7.1 This
                            system is not configured to relay mail
from
                            <[log in to unmask]> to
                            <[log in to unmask]> for 332.222.1.21"


Again, can anyone explain in layman's terms what is happening here?
(One more coming...)

Thanks again,
Christine in Littleton, Massachusetts