ZCS Connector for Outlook Performance

ZCS Connector for Outlook

   KB 2905        Last updated on 2015-07-13  

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There are a number of factors that can affect the performance of the Zimbra Connector for Outlook (ZCO). These include general issues, the use of virus scanners, desktop search, Outlook AddIns, GAL (Global Address List) synchronization, and the use of logging. Some future enhancements will also address performance issues.

General Outlook Issues

There are a number of things people can do to improve the performance of Outlook, and therefore ZCO. Hardware configuration can have a direct effect on Outlook performance. Of course, a machine with large amounts of memory and a very fast disk drive will have better performance. However, except for initial zdb loading, Outlook is not particularly memory intensive. It is, however, quite disk intensive, so getting a high RPM disk can improve performance quite a bit.

Many users have a number of Outlook add-ins running. It has been shown that having a large number of add-ins, and having particular add-ins can slow Outlook down. Users should uninstall or disable any add-ins they are not using.

A number of articles have been written detailing the degradation in performance when Outlook mailboxes increase in size. Various sizes have been mentioned when talking about a drop-off in performance, including 2GB, 4GB, and 7GB. It is best to keep mailboxes smaller, if possible. ZCO has a Compact ZDB… button on the ZDB Configuration tab. For more information on issues with large mailboxes, please see http://support.microsoft.com/kb/932086.

Outlook 2007 SP2 delivers a number of performance improvements, particularly in the areas of startup and shutdown. ZCO users have noticed shutdown issues in the past. Prior to SP2, Outlook would not shut down if it detected a third party was accessing it, which frequently prevented it from exiting altogether. SP2 changes this behavior by stopping all server activity and no longer waiting for external references to be released. ZCO Shutdown issues have virtually disappeared with the use of SP2.

Virus Scanners

Virus scanners can significantly slow down ZCO performance. NOD32 has been identified as not working well with Outlook 2007. AVG is a known culprit, particularly version 8. In many cases these scanners can render HTML E-mail as plain text. AVG’s certification feature appends text to mail messages, sometimes causing these messages to lose HTML formatting. To disable this, go into AVG’s Advanced Settings, and under E-mail Options, uncheck the box that deals with Certification of Incoming E-mail. Sometimes virus scanners render emails completely blank.

Many anti-virus users disable E-mail scanning, since Zimbra does it for you. To disable the E-mail scanner in AVG, open the Control Center, select the E-mail scanner plug-in, click Properties, click on the Disable Plugin button, and check Ignore Plugin Condition.

Desktop Search

In Outlook 2007, the Instant Search functionality requires a component called Windows Desktop Search. This component can cause performance problems for ZCO. Many people uninstall this entirely. If this program is a necessity, you should turn off indexing for .zdb files. To do this, click Tools, Instant Search, Search Options, Indexing. Uncheck the .zdb file.


There are a number of addins that cause syncing problems with ZCO. With these addins, syncing can sometimes just stop for no apparent reason. We have identified 2 addins that can cause this problem: the iTunes Outlook Addin and the Outlook Change Notifier. In version 5.0.12, we implemented an enhancement (bug 32969) that detects the presence of these 2 addins, notifies the user, and disables them. There are 2 registry keys that have the addin list, so that users can customize which addins are to be disabled:




Many users have very large Global Address Lists, and this can cause performance problems on initial sync. We have done a number of things to address these issues. These were all done in ZCO version 5.0.12.

  • Bug 32571.This enhancement allows users to disable the GAL sync entirely.
  • Bug 34765.This enhancement added a manual GAL sync mode, so users can download the GAL at their convenience.
  • Bug 34766.This enhancement added the UI for GAL sync modes.
  • Bug 34781.Among other things, this allows the user to customize the installer to set the GAL sync mode.
  • Bug 29429.This enhancement implements GAL sync throttling. This allows the user to control how much CPU the GAL sync uses. It is controlled by a registry value: HKLM\SOFTWARE\Zimbra\GalFullSyncSleep. Note that this same value is available for in HKCU for user customization. This value contains the number of milliseconds to sleep while doing a full GAL sync. Every 10 contacts, we will sleep for this amount of time. The range is between 1 and 1000. If the value is 0 or not there, we will not do GAL sync throttling. The default is no throttling, although this can also be customized in the ZCO installer (bug 34781).


Note that ZCO logging (especially verbose logging) can cause significant performance degradation, especially on initial sync. Unless there are issues, we recommend that ZCO logging be turned off for initial sync. Additionally, the Calendar Tracking logging feature can cause performance problems as well. This is a debugging tool, and should be used only for that purpose.

Delegate Stores

Delegate stores are implemented in ZCO as separate .zdb files that are mounted in a given profile. When a user shares a calendar or other folder, the grantee accesses the share by mounting a delegate store. Outlook incurs a lot of memory overhead when mounting PSTs (.zdb) because it allocates a large in-memory cache in order to facilitate random reads and writes. Unfortunately, this means that it can be allocating 15 to 20 MB per mounted delegate. Mounting many delegates at a time can hurt performance significantly, especially on machines without much memory. Outlook 2007 helped somewhat with this, but it is still a big issue. We recommend that you only mount those delegate stores that you absolutely need.


We have implemented and will implement a number of enhancements to improve performance. These include downloading only headers and not entire mail messages, downloading only certain folders, and changing the order that items get downloaded.

Headers Only

This is bug (enhancement) 11962, and is available in ZCO version 6.0.0. This allows users to configure a profile so that on a sync, we download only the message header, and not the entire message. If the user wishes to download the contents of a message, he or she right clicks on the message to mark it for download, and on the next sync, the contents of the message will be downloaded. This can increase performance by as much as 90% on initial sync. To enable this feature, when configuring a ZCO profile, go to the Download settings tab, and check Download headers only.

Folder Sync Options

This is bug (enhancement) 12790, and this should be available in ZCO version 6.0.1. Many times, users do not need to sync all their folders. This enhancement gives the user a choice of which folders to download, thereby increasing performance by syncing much less data.

Initial Sync Order

This is bug (enhancement) 26157, which will be available in a future version of ZCO. This enhancement changes the order of syncing folders, providing the user with more relevant messages first. Specifically, recent inbox messages and calendar appointments will be available to the user immediately.

Verified Against: ZCO 5.0.x Date Created: 6/19/2009
Article ID: https://wiki.zimbra.com/index.php?title=ZCS_Connector_for_Outlook_Performance Date Modified: 2015-07-13

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