CiviMail should recognise more than just the first email address of the primary location

Currently, CiviMail operates only on the first email address of the primary location of a contact. This means that (a) a mailing going to a contact will get mailed to the first email address of their primary location and (b) if a contact tries to subscribe to a group with any email address other than the first email address of their primary location, a new contact will be created (instead of the proper contact being recognised and acted upon).

As a minimum, all other email addresses of any contact should be recognised as associated with this particular contact.

Ideally, any email address of a contact should be flaggable as the address to send bulk mail (a sensible use case: a person with an email address snoopy@example.com might want to use snoopy+lists@example.com as their bulk mail address).