Many entrires in amavis: amavis-20071101T175449-21286

Again dual sendmail with amavis and spamassassin.

I often have a big list of old entries in amavis-
spool area like "amavis-20071101T175449-21286".

Are those entries necessary ? Can they be removed ?
Have those Mail-Parts already be processed by
amavis ? What can be the reason for it ?

Thanks Harald