Strange Problem that has nothing to do with BIND or DNS at all

[Jonathan de Boyne Pollard]

B> Does anyone have any ideas on why local sendmail clients 
B> fail to deliver to the backup mail server?

Yes.  I have.  I'm sure that the denizens of the "comp.mail.sendmail"
newsgroup have, too.  Ask them why your fallback mail exchanger isn't used
when you inject mail either on your mailhost or on other machines that are
configured to send everything to a mailhub.