Major Description
Currently our getRecipients expose the internal collection and it is used that way.
To add recipients we use getRecipients().add(), to store multiple times a single mail to different recipients we get the recipients collection and alter it between store of the mail object (in RemoteDelivery).
All of this seems to me bad practice and we should better encapsulate the recipients list in the mail object.
Maybe the simple way is to return immutable collections so we'll find bugs as soon as possible and add methods to add and remove recipients directly in the mail interface.