error: Host unknown, please help!

[Jeremy]

Greetings,

I'm having a heck of a time figuring this out....  I'm a little fresh with
Sendmail and Bind, too...

Checked the FAQs to no avail.  The problem is as follows.

Our web host company is hosting the website, but we are hosting our own
mail.  The mail was transferred to a new box (from Eudora/Mac to Sendmail
8.9.3 on RHL) because the company moved and changed IP.  The webhost company
is forwarding out domain mail to this box.... I think.

Whenever I send a message to mail to user123 at mydomain.com I get a return
response from *somewhere* with the "Transcript" of:

550 mailto:user123 at mydomain.com...  Host unknown (Name server:
my.ip.add.ress.mydomain.com.: no data known)

Inspection of the headers lead me to believe the message never gets to our
RHL/Sendmail box... because the last stamp is stated simply:

localhost (localhost) by my_webhost_provider.

Any help so greatly appreciated!

-jeremy