Newbie Topology Question

Hi,

What is the standard approach for a small-medium sized company for setting
up a mailserver for internet based applications?

Should a webserver that hosts an application that requires SMTP access have
a mailserver in the same physical network or can the companies internal
mailserver be used.

I am leaning towards having a dedicated mailserver on the network that is
exposed to the internet.

Any help/advice or best practive papers is greatly appreciated.

TIA

MattC