Text markup and presentation seems like an end system application. I can't
see any advantage to having it done as part of delivery. It would make
a lot of sense to have the marked-up text available for translation into
several different formats (pdf, word, html) even after the initial delivery,
so it does not seem like an OPES application.
Anti-spam, though, seems like a good OPES application. It involves delivery,
it involves using an existing protocol (SMTP) embedded inside another
protocol state machine, it only needs to be done once as part of message
delivery.
Hilarie