Zimbra uses a standards-based system. For example, email uses the IMAP standard, calendars use the .ICS and CalDav standards, and contacts support the vcard/vcf (including auto-adding to your contacts from attached vcards!) standards, as well as import/export via CSV (including contact export for various systems, including Thunderbird, Outlook, Gmail, Yahoo!).
Furthermore, the Zimbra Free-Busy Interop supports two-way sharing of free/busy information, schedules, and global address
book (GAL) access with Microsoft's Exchange Server, IBM's Lotus Domino
Messaging Server, Meeting Maker, and a slew of other third-parties that
interact with the Zimbra free-busy API framework.