The mailbox module uses the rfc822 module as its
default factory for creating message objects. The
rfc822 documentation claims that its use is deprecated.
The mailbox module should probably use the new email
module as its default factory.

Of course this has backward compatibility issues, in
which case it should at least be mentioned in the
mailbox documentation that it uses the deprecated
rfc822 module, and provide an example of how to use the
email module instead.