All email send to a account on my raq3 is bounced with the following error:

----- Transcript of session follows -----
procmail: Renaming bogus mailbox
"/home/sites/site67/../../spool/mail/username" into
"/var/spool/mail/BOGUS.g.resete._iF" procmail: Couldn't create
"/home/sites/site67/../../spool/mail/username" 550
<username@domain.nl>... Can't create output

Where username is the actual username and domain the domainname.

How can i solve this ?

looks like a disk quota issue, check site usage and user quota.