OPES is supposed to enable new services.
Absolutely.
The call-forward wouldn't have been a hack if it had been done as
part of an OPES service, using the same architectural model that
we used for HTTP.
I fully agree.
Further, we rule out a large number of useful
identity-based services by eliminating the ability to do
response modification.
Here I disagree.
Tonny describes again a use case that we've listed already last year.
See sample 2-2 on http://www.martin-stecher.de/opes/smtpusecases.html
No doubt that it is a very important one.
This and other examples of group 2 are the "useful identity-based services"
you are referring to, aren't they?
And these are all request satisfaction examples, not response modification.
Again: Response modification is only useful if the callout service makes
use of the original response data, which in SMTP is the SMTP reply.
The only service I can think of, that does this, is a logging service.
Is that one strong enough to have response modification in the SMTP deployment?
Or can you please describe another important service that makes use of
the response and modifies it?
Regards
Martin