temporary verify error

Hello,

I have been seeing in my qmail-smtpd log file many "temporary verify  
error:" related with RCPTCHECK. When it occur, the RCPTCHECK always  
consider that user doesn't exist and bounce the mail. The problem is  
that many mails for existent accounts are being bounced. The problem  
happens aleatory.

Any idea about what can be happen?

Regards,
FJ