Content filtering for Local delivery mailboxes- how to?

[blason]

Hi Folks,

Here is my scenario and would like your opinion on email routing. I have an internal domain say example.com, mails coming from Internet are getting scanned through perimeter content filtering application while for local delivery would like to know how can I route all those emails to either the same content filtering box or if I can put other content filtering box would need to route the mail through that before it makes to delivery to mailbox.

i.e two user say foo@example.com and bar@example.com, if they send mails to each other how can that be passed through other content filtering box?