VMware Zimbra Collaboration Server (ZCS) is a full-featured messaging and collaboration solution that includes email, address book, calendaring, tasks, and Web document authoring

The ZCS architecture is built with well-known open source technologies and standards based protocols. The architecture consists of client interfaces and server components that can be ran in a single node configuration or deployed across multiple servers for high availability and increased scalability.
Open source integrations. Linux, Jetty, Postfix, MySQL, OpenLDAP.

Uses industry standard open protocols. SMTP, LMTP, SOAP, XML,IMAP, POP.

Modern technology design. HTML5, Javascript, XML, and Java.

Horizontal scalability. Each Zimbra mailbox server includes its ownmailbox accounts and associated message store and indexes. Zimbra has the flexibility to scale both vertically by adding more system resources orhorizontally by adding more servers.

Browser based client interface. Zimbra Web Client gives users easy access to all the ZCS features.

Browser based administration console.

Zimbra architecture:

Includes open-source integrations using industry standard protocols. The third-party software listed below is bundled with Zimbra software and installed as part of the installation process.
Jetty, the web application server that Zimbra software runs in.

Postfix, an open source mail transfer agent (MTA) that routes mail
messages to the appropriate Zimbra server

OpenLDAP software, an open source implementation of the Lightweight Directory Access Protocol (LDAP) that stores Zimbra system configuration,the Zimbra Global Address List, and providers user authentication. Zimbra can also work with GAL and authentication services provided by external LDAP directories such as Active Directory

MySQL database software

Lucene, an open source full-featured text and search engine

Anti-virus and anti-spam open source components including:

ClamAV, an anti-virus scanner that protects against malicious files
SpamAssassin, a mail filter that attempts to identify spam
Amavisd-new interfaces between the MTA and one or more content checkers

James/Sieve filtering, used to create filters for email

ZCS includes the following application packages.

Zimbra Core:
Includes the libraries, utilities, monitoring tools, and basic configuration files.zmconfigd is part of zimbra-core and is automatically enabled and runs on all systems.

Zimbra LDAP:
ZCS uses the OpenLDAP software, an open source LDAP directory server. User authentication, the Zimbra Global Address List, and configuration attributes are services provided through OpenLDAP. Note that the Zimbra GAL and authentication services can be provided by an external LDAP Directory such as Active Directory.

Zimbra MTA:
Postfix is the open source mail transfer agent (MTA) that receives email via SMTP and routes each message to the appropriate Zimbra mailbox server using Local Mail Transfer Protocol (LMTP).The Zimbra MTA also includes the anti-virus and anti-spam components.

Zimbra store(mailbox server):
The Zimbra store package installs the components for the mailbox server, including Jetty, which is the servlet container the Zimbra software runs within. Within ZCS, this servlet container is called mailboxd.

Each account is configured on one mailbox server, and this account is associated with a mailbox that contains all the mail messages, file attachments, contacts, calendar,and collaboration files for that mail account.Each Zimbra server has its own standalone data store, message store, and index store for the mailboxes on that server.As each email arrives, the Zimbra server schedules a thread to have the message indexed (Index store).

Zimbra uses swatch to watch the syslog output to generate SNMP traps.

The Zimbra logger installs tools for syslog aggregation,reporting. If the Logger is not installed, the server statistics section of the administration console is not displayed.

A spell is the open source spell checker used on the Zimbra Web Client. When zimbra-spell is installed, the Zimbra-Apache package is also installed.

Use of an IMAP/POP proxy server allows mail retrieval for a domain to be split across multiple Zimbra servers on a per user basis. The Zimbra Proxy package can be installed with the Zimbra LDAP, the Zimbra MTA, the Zimbra mailbox server, or on its own server.

It is a separate package from zimbra-proxy and is automatically selected when the zimbra-proxy package is installed. One server must run zimbra-memcached when the proxy is in use. All installed zimbra-proxies can use a single memcached server