[Dovecot] help on writing a rule for perventing spam

Hi I used qmail + dovecot-1.1.7 + dovecot-sieve + .... now everything works well but there are many spam in Bulk folders from every user address to their address for example from user1@mydomain to user1@mydomain in the real the sender and reciever are the same . they are spam but since everyday , everyvirtual user has many of these spams in their Bulk i need a rule in dovecot.sieve to prevent them Can anybody help me for writing this rule.

with regards Sophia Alikhani WorkPhone: +98-21-8497057 Mobile : +98-912-3361036