The from_address method is added to Mail::Message, but is not used
internally to ActionMailbox, nor tested (even implicitly). As it is part
of the public API, it feels important to at least have a basic test of
it.
Similarly, the x_original_to_addresses was only implicitly tested via
the recipients_addresses method. Given other similar methods are
explicitly tested, it feels like an oversight not to test it as well.
As the test introduced from_address didn't align with the naming of
RecipientsTest, I renamed it to the more generic AddressesTest.
