[Exim] Error 451 problem with squirrel mail and cyrus IMAP

I have built a system that uses exim/cyrus/squirrel.

I can send and recieve email using pop3/smtp with outlook express so the
that side os ok. I can also login in to squirrel using cyrus/IMAP/sasl and
see email however when I try to send email from squirrel I get the
following :-

"Requested action aborted: error in processing
Server replied: 451 Temporary local problem - please try later"

why does this happen ? and what is the most likely solution ?

Thanks

Ian Warburton