Intranet address rejected

We have an intranet system creating automatic messages from web pages.
We use the sendmail feature of the intraserver which has a private
IP-address. recently the mail messages have been jammed in the mail
que. There is a status message in the mail que of sendmail:

(Deferred: 450 4.1.8 : Sender a)

The receiving mail box is of course in the public internet. So I
understand that the POP/IMAP server is rejecting the message due to the
unresolvable sender address.

How could I have the sender information be shown acceptably. And why is
it not showing the sender given in the message headers (FROM:)?

We have static IP addresses so DNS entries could maybe used. Being
quite ignorant of network administration issues I would need detailed
instructions.

Thanks for help,
Harri