Mahogany has a built-in address book system with flexible support for different addressbook formats. It can easily expanded to handle other programs' file formats and at present supports two formats, it's own ADB format (1.8.1) and (X)Emacs' BBDB (1.8.1) address book format. Generally the address books are ordered hierarchical, with the possibility to order entries in groups and sub-groups in a tree-like fashion. Alias expansion is configurable and also handles group names, to expand to the full list of email addresses within that given group.