There are a few places in the new email API where dicts are returned containing what is conceptually static information.  Currently this is done by returning a copy of the dict from the object, so that user code modifying the dict won't break the object invariants.  It would be better to change these to MappingProxyType objects instead, before the API moves out of provisional status.

This issue is mostly a note to myself, since I'm the most likely to be able to figure out which places in the code need changing, but if anyone else wants to look at it feel free, since it will probably be a while before I get to it.