HPUX 11.23 on rp7410

We just noticed that our HP server is not receiving (rejecting?)
emails from outside our network. We can send emails internally and
the HP server will get the emails, however if we send say from
yahoo.com the emails never arrives and most of the time we don't get a
bounce messge. When we do bounce messages we seem to get 4.4.1 and
5.1.1 errors. But most test emails we never get a bounce message.

We use exchange as our primary mail service and have links with our HP
& linux server via exchange. The exchange logs say the message was
delievered. but we don't see it on the HP.

My guess we have something mis-configured on sendmail, anyone have
ideas of where to start looking?

We have no idea how long this has been happening, but looks like a for
a while.