slightly OT - my freebsd email topology

[Jonathan Horne]

currently, my email server is just a single box, accepting and sending emails
from and to the internet.  spamassassin and sendmail, and so far, it works
satisfactory.

i would like to change it up, so that i have a pair of servers doing MX from the
internet, which then passes to an internal server for delivery.  if i do that, i
could remove spamassassin from the internal server, and run it on just the 2
external.  all those configurations is really not my issue here... what im
really pondering is how would external servers that are seperate from where the
target mailboxes are, know which addressess are acceptable and which to return a
550?

does anyone have any setups that are similar to this, and could advise me or
point me in the right direction?

thanks,
jonathan