The agent (which in reality is a Resource in order to be able to own
Collections) creates Inbox, Sent and Drafts virtual Collections and links
all Items from real special Collections into them. It listens for changes
in the source collections and updates the virtual Collections accordingly.
Uses can also customize the default unified mailboxes and create their
own.
The agent can be enabled or disalbed from KMail's Plugins settings like
other agents. By default the agent is off, however, if KMail detects that
user has more than one account set up it will ask the user if they want
to enable the feature. User is only ever asked once, the purpose is to
make the feature easier to discover without bothering users with only
one email account.
It appears that the virtual collections are slower to open than regular
collections, but that's a problem on Akonadi side as querying Items from
virtual collections requires some extra queries. Other than that it works
pretty well.