[Exim] 451 Error in writing spool file ?

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Hi:

    Some user will get error message reply when sending mail
   through our server:

    "451 Error in writing spool file"

    I checked the exim source and guess maybe the reason is
   some problem when fflush data_file. 

    And I found our spool db files is seem "huge" when "ls -l".
   (wait-remote_smtp.pag is 65G, but 3824 blocks)

So I try to rm all db files and everything is ok until now.
And no more "451 Error in writing spool file".

    Does anyone know the real reason ?

    Thank you very much...

( Our os is Solaris 2.6 )

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Ching-Wei LIN
cwlin@???.