Split Domain

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We are often asked how to configure the Zimbra Collaboration Suite so that some accounts are moved/migrated to Zimbra but other accounts continue to live on the existing mail system, in what would be a Split domain. This is possible if your mail system offers some of the mail routing features that Zimbra offers.

Consider this example:

  • Your domain is example.com.
  • Your existing mail infrastructure lives on the host "mail.example.com" (assuming single node install for simplicity, though what we are discussing should also work on multi-node).
  • Your domain example.com has MX records that to point to host "mail.example.com".
  • Users "foo@example.com" and "bar@example.com" exist in the old system.
  • You have installed Zimbra on the host "zimbra.example.com" and created the domain "example.com" on it also.
  • You have migrated the mailbox of the user "foo@example.com" to the Zimbra system, using one of our migration tools documented elsewhere.

Some issues to consider as a result of such splitting your domain

  • The end user "bar@example.com", who is not being migrated, will be unaware of the change insofar as regular email is concerned. Global address list (GAL) experience should also not change for this user. There may be some issues around group calendaring between the two systems (assuming your existing infrastructure supports group calendaring); we are working on these issues.
  • The end user "foo@example.com" will need to be educated and the user's desktop software might need some minor configuration changes. Users accessing mail through POP3 or IMAP will need to modify POP3/IMAP server settings to point to the Zimbra server. Users accessing mail with Outlook will need the ZCS Connector for Outlook installed and configured to access the Zimbra server. Users who will access mail with the Zimbra Web Client will need the hostname (and port number when services run on non-standard ports) of the new Zimbra server.
  • If the user "foo@example.com" is using a mail client that has outgoing SMTP configured with SMTP AUTH enabled, and SMTP AUTH functionality is required by your network topology or roaming users, then based on where authentication information lives (a decision you will make below), it may be necessary to change the user's outgoing SMTP setting.
  • You will have to pick either the Zimbra system or your existing system to be the "authoritative" system for your domain. Let's call the authoritative system the primary system, and the other one the secondary system. The primary MTA must be aware of all accounts on the domain, and must reject on RCPT TO: those accounts that do not exist in either system. The secondary MTA must accept mail for accounts hosted on the both the secondary and primary systems, and must forward mail for all accounts on the domain hosted on the primary system to the primary MTA.
  • Note that because of the above primary/secondary mail routing requirement, it will be necessary to perform some type of provisioniong step on both the secondary and the primary for an account that lives on the secondary system.
  • For ease of administration you should keep any Global Address List (GAL) on only one of these systems to avoid duplication. However this requires one these systems to be able to proxy to the other for GAL (Zimbra can use a non-Zimbra external LDAP server for GAL (Admin Guide p.50)). If this proxy is not possible some other sync method will need to be developed for consistend GAL access across both systems.
  • Zimbra can also use an external LDAP server to authenticate users (Admin Guide p.51). You may, for example, have a portal or user page backed by LDAP where passwords are stored. Note that by default Zimbra stores user passwords locally in the Zimbra LDAP. If passwords are managed remotely, it is possible the old system can continue to SMTP authenticate users who live on the Zimbra system. If that is the case, then a Zimbra user with SMTP AUTH can continue to use the old system's MTA as their MTA, assuming it is visible on the network.

How to provision and configure Zimbra as either the primary or as a secondary system.

If you decide to use Zimbra as the primary system, then you should also carefully read the section on how to configure Zimbra as the secondary system - this will show you what functionality is required of your existing mail system so it can be the secondary. The same is true vice versa if you are going to use Zimbra as the secondary system.

Configuring Zimbra as the Primary System

1. Create corresponding accounts on the Zimbra system for all the accounts that will live on the secondary system. Note that bar@example.com is not migrating, and lives on the secondary.

$ zmprov ca bar@example.com <some_random_password>

2. Configure mail routing for this account so email flows to the secondary system for this account:

$ zmprov ma bar@example.com zimbraMailTransport smtp:mail.example.com

Note that we are not using any catch all (akin to "luser_relay") tricks here. Primary needs to be authoritative, and if it forwards unknown accounts, then we would have a mail loop.

3. Change your MX record so mail from the internet flows into the Zimbra MTA first. (This is the last step! You will bounce mail if you make this change before configuring the entire system and testing that mail flow is working as desired.)

When you are ready to move a user from the old system to the new system just run this command (where zimbra.example.com is the name of your Zimbra server):

$ zmprov ma bar@example.com zimbraMailTransport lmtp:zimbra.example.com:7025

Configuring Zimbra as the Secondary System

1. The secondary system needs to accept mail for accounts that are hosted on the secondary, but must forward all other mail for accounts on this domain to the primary system.

You can accomplish this piece of mail routing with these commands:

$ zmprov md example.com zimbraMailCatchAllAddress @example.com
$ zmprov md example.com zimbraMailCatchAllForwardingAddress @example.com
$ zmprov md example.com zimbraMailTransport smtp:mail.example.com

The first two commands (in combination) tell the Zimbra postfix to accept all addresses in the @example.com domain as valid addresses.

The third command establishes default mail routing for the domain.

2. We also highly recommend that, in a secondary Zimbra system, you turn off DNS lookups and internet wide message routing from the secondary host and route all mail through the primary. You can accomplish this by:

$ zmprov mcf zimbraMtaRelayHost mail.example.com
$ zmprov mcf zimbraMtaDnsLookupsEnabled FALSE

Make sure to configure mail.example.com to accept mail relayed by zimbra.example.com.

3. After completing the above configurations, run these commands:

$ postfix stop
$ postfix start

Examples of Mail Flow

Case 1

  • Zimbra is the primary system
  • Mail is sent from the internet
  • The account lives on the secondary system
:internet ->
:zimbra.example.com postfix server ->
:account transport says to use smtp:mail.example.com ->
:mail.example.com existing infrastructure MTA ->
:mail.example.com existing infrastructure mail store

Case 2

  • Zimbra is the secondary system
  • Mail is sent from foo@example.com (hosted on zimbra) -> bar@example.com (hosted on the old system)
  • foo@example.com is using the Zimbra AJAX client
:foo's web browser ->
:zimbra.example.com tomcat server ->
:zimbra.example.com postfix server ->
:account not local, finds domain transport setting ->
:mail.example.com existing infrastructure MTA ->
:mail.example.com existing infrastructure mail store
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