[Exim] Spool error?

Hello,

I've recievd this error on one of the machines I'm an admin for:

Server Rejected Message, 451 error while writing spool file

/var/spool/exim/input is 1.3 Gigs, with permissions set:

drwxr-x---    2 mailnull mail

It does appear to have 150,000 messages in the spool, but most users
can send mail. Any idea what could casue this issue? I've googled
around for many of the 451 errors but havn't found much information on
them.

Thanks for the help,

Erik Baar