Simple Workflow

* Inbound
Internet -> ASSP -> Mailserver -> User

* Outbound
User -> ASSP -> Mailserver -> Internet


Advanced Workflow 
- recommended for installations with considerable number of users 
- necessary for MS Exchange & similar workgroup server)

* Inbound
Internet -> ASSP -> SMTP Relay Server -> Mailserver -> User

* Outbound
User -> Mailserver -> ASSP -> SMTP Relay Server -> Internet

inbound traffic coming from the Internet is proxied by ASSP
toward the additional local SMTP Relay Server which in turn routes the emails to the
internal mailserver; this also has an advantage; if you have more than
a single "internal" mailserver (e.g. different mailservers for different
domains) such a setup will take care of routing the incoming emails
to the correct mailserver;
outbound traffic is routed by the mailserver(s)
toward the ASSP relay port (which may even be port 25 as long as the
ASSP box has a second address ) and ASSP then proxies such emails
to the SMTP Relay Server for outbound delivery