This module provides an API for email related modules, and a hook system for mail server related modules. The name is a little bit misleading since the module could hypothetically access other message stores as long as they provide message IDs. For this release IMAPWU API has been moved into contrib directory so it's a part of Mail API now.
This also contains a sub-module called mail_api_service, which uses the Services module to provide a service that can increase the flexibility of this module even further. This part of the module is lagging behind a little bit while I'm trying to stabilize Mail API itself.