We are trying to convert emails to user_profiles earlier in
the codepath. This may cause subtle changes in which errors
appear, but it's probably generally good to report on bad
addressees sooner than later.
This class simplifies the calling sequence to methods like
check_message and _internal_prep_message, and it's also more
type safe.
Checking for message types is encapsulated with calls to is_stream()
and is_private(). There are also shortcut constructors when you
know that the type of the address (stream vs. private), which is often.