I have not. Do you have SpamAssassin running... If so, how long do you have Delivery Delay set to
(Setting -->General Settings --> Spool ). Never have lost any emails.
As for RBLS.. we have what ever Bruce has recommended or outlined in his spam documents. (which is quite a few).
What do you have running in the back-ground that would be accessing the spool.
Which logs are giving you those messages???