You need to create a secondary smtp addresses.
If only one user needs to receive sales@domain.com e-mails, go to your recipients container, select your user, select e-mail addresses tab and add a new internet address.
If more than one user needs to receive these mails then create a distribution list, give it the smtp address of sales@domain.com then make all required users a member of the list.
If sales@domain.com is a mailbox that already exists, go to the mailbox, select delivery options and set an alternate recipient.
Hope this helps.
Wayne