External & Internal mail server

[Vu Pham]

Some of the documents I read about firewalls mention about internal mail
server and external mail server in the DMZ and suggest to let the employee
to access to the internal mail server which will forward outgoing email to
the external mail server which will forward incmoing email to the internal
one.

Why do we need two ? Why don't we use only one mail server put in the DMZ
zone ?

Thanks,

Vu