sendmail hostname problem

I have a SUSE 7.2 sendmail vanilla config. (out of box) It's hostname is mail.seventythirdstreet.com My dns is set for seventythirdstreet.com so that the mx record points to mail.seventythirdstreet.com When I send mail to peter@mail.seventythirdstreet.com it works fine. When I send mail to peter@seventythirdstreet.com I get a relay error as follows: <peter@seventythirdstreet.com>: 64.81.61.39 does not like recipient. Remote host said: 550 5.7.1 <peter@seventythirdstreet.com>... Relaying denied Giving up on 64.81.61.39. Any ideas? Thanks