1. Add the following lines into Postfix master file (master.cf).
    
filter    unix  -   n   n   -   -   pipe
    flags=R user=drweb argv={drweb-directory}/drweb-postfix -f ${sender} -- ${recipient}
    
    or   

filter    unix  -   n   n   -   -   pipe
   flags=R user=drweb argv={drweb-directory}/drweb-postfix --conf={/path/to/your/conf/file} -f ${sender} -- ${recipient}

    attention: {path/to/conf/file} should contain filename too        

2. Give Postfix a direction to check all the letters incoming through SMTP.
For that you should find the following record in Postfix master file
(parameters n - n - - may be different):

smtp          inet  n       -       n       -       -       smtpd 

and change it into:

smtp          inet  n       -       n       -       -       smtpd -o content_filter=filter:dummy