resolve problems

[zippy the chimp]

I am experiencing trouble with DNS/possibly sendmail.

the best way to describe it, is that we have a name.com address.

when user at name.com sends mail it gets returned by the remote host
saying that "Sender Domain Must resolve" and others are having trouble
sending mail to us.

when i do a nslookup up host1.name.com it resolves fine, but when i do
a nslookup on name.com it returns nothing, what would you guess i
need to change.
PS. when i do a nslookup on mindspring.com i get a
bevy of servers that are returned but my domain has none.

thank you for your help.