Forwarding incoming mail from outside

Hello,
I have an exchange server on SBS2003. For certain users I would like to forward the incoming mail from outside the organisation to the departement mailbox. The problem is that I don’t want this forwarding for mails sended inside the organisation. For example if somebody from outside the organiation send a mail to the user John Smith, this mail have to be forwarding to the mailbox (without copy to the user) of the sales departemnt. But If another user of my organisation send a mail Jo he must receive this in his mailbox. If it can help, each user have to mail adres one with domain.local and the other with domain.com.
Please, help ! Thank’s !