email components faq?

["Bernd Prager" <bernd@prager.ws>]

I would like to catch up on some mail component techniques and vocabulary.
I am getting confused with meaning and interaction of all the different
things like:

- mail user agent (like mailx)
- mail delivery agent (like maildrop)
- mail transport agent (like postfix)

Does anybody know a somehow comprehensive FAQ that can help me
understanding what everything is doing and how they're supposed to
interact?
I've been searching for a while, but all I found is already too package
specific.

Thanks,
-- Bernd