You are operating an MTA business service.
You offer customers a red star on their outgoing email, and a blue
star on incoming email.
Your SMTP service sends outgoing email to the OPES callout server with
a "red star" service request. The email comes back from the callout
server and is delivered to its destination.
Your SMTP service sends incoming email to the callout server with a
"blue star" service request. The email comes back from the callout
server and is delivered to its destination.
For anything more complicated, try using the terminology in
the OPES architecture document.
Hilarie