Integration & Customization
GAL feature tuning: Global Address List (GAL) is configured on per-domain basis via special system accounts (aka galsync accounts).
Theme/Skin configuration: Customized themes (skins) can be created through the administration console as a global setting or as a domain setting.
Directory integration for authentication: Active Directory (AD) can be easily configured using the admin console. AD can be used for authentication and objects (conference rooms, etc).
Provisioning & Migration
Bulk provisioning / Mailbox management: Command files and scripts (zmprov / SOAP APIs)