[Exim] Unable to send messages using exim3.22

Hi

I am running exim3.22 on a sun server running Solaris 2.8
It compiles and runs fine, however when I try to send a message I get
the following error

"    451 rejected: temporarily unable to verify sender address (try
   again later) <sherona@???>

Any ideas as to what could be causing the problem.

Thanks
Cheers
sherona
Computer and Network Services
University Of The Witwatersrand, http://www.wits.ac.za
Johannesburg
South Africa