The User Mailbox contains entities that are private to the user. This is in contrast to TMails which are shared entities. The User Mailbox helps you to organize your TMails, follow up on them, organize your Drafts and Response Drafts.

The User Mailbox entities are organizd in different folders and these appear in the Navigation bar, as below. (Please see Navigation for more details.) 

The main entities in the User Mailbox are TMail Pins, Messages, Tags, Drafts and Response Drafts.

TMail Pins correspond to a TMail. Messages correspond to a specific Changeset within a TMail. The figure below illustrates these relationships