[Exim] SMTP policy issues - where to take them?

Is there a mailing list that deals with SMTP policy issues, or
are there other ressources in that direction (documents, FAQs
and the like?). What do you recommend?

I am thinking about topics like "What is better: A cluster with
a single MX, or multiple machines with separate MXes?" "What
trust can be put into the name that comes after a HELO
statement? Can we demand that it resolves?" "If my secondary
gets a mail while the primary is reachable from the secondary,
should the secondary 4xx the incoming message?"

Where would I take questions like these?

Kristian