How to Optimize Outlook for Long-Running Citrix Published App Sessions

When you run Outlook as a published app in a Citrix environment, you may notice slow performance, frequent disconnections, or high memory usage over time. These issues typically stem from Outlook’s default settings, which are designed for local or short-term remote use. This article explains how to adjust Outlook’s caching, add-in, and connection settings to maintain stable performance during long-running Citrix sessions. You will learn specific registry edits, profile configurations, and group policy settings that reduce resource consumption and prevent session degradation.

Why Outlook Slows Down in Long-Running Citrix Sessions

Citrix published apps run on a shared server or a virtual desktop that persists for the user. When Outlook is left open for hours or days, its default behavior of caching email, indexing search data, and loading add-ins accumulates memory and processor overhead. The problem is compounded because Citrix sessions share system resources with other users, so any memory leak or high CPU usage affects the entire server.

The root cause is Cached Exchange Mode. By default, Outlook downloads a copy of the mailbox to a local OST file. Over a long session, the OST file grows, and Outlook continues to sync changes with the server. This constant read-write activity increases disk I/O and memory usage. Additionally, Outlook’s search indexer builds and updates a local index, which consumes CPU cycles even when the user is idle. Add-ins, especially those for third-party line-of-business applications, often leak memory or cause Outlook to hang after extended use.

Steps to Optimize Outlook for Citrix Published Apps

1. Switch to Online Mode
   Open Outlook. Click File > Account Settings > Account Settings. Select your email account and click Change. Under Offline Settings, uncheck “Use Cached Exchange Mode.” Click Next, then Finish. Restart Outlook. This forces Outlook to read data directly from the Exchange server without creating a local OST file. The session uses less memory and disk space.
2. Disable the Search Indexer
   Open Control Panel > Indexing Options. Click Modify. Uncheck all Microsoft Outlook locations. Click OK. This stops Windows Search from indexing Outlook items. In long sessions, the indexer can cause high CPU usage and slow down the published app. Users can still search using server-side search in Exchange Online or Exchange Server.
3. Remove Unnecessary Add-ins
   In Outlook, click File > Options > Add-ins. At the bottom, select COM Add-ins from the Manage dropdown and click Go. Uncheck any add-ins that are not required for core email functions. Common non-essential add-ins include Skype for Business, LinkedIn, or third-party CRM connectors. Restart Outlook. Fewer add-ins mean fewer processes running in the session.
4. Disable Hardware Graphics Acceleration
   Click File > Options > Advanced. Under Display, check “Disable hardware graphics acceleration.” Click OK. This reduces GPU resource usage in the Citrix session. It also prevents rendering issues that occur when Outlook is left open for long periods.
5. Set Outlook to Send Immediately
   Click File > Options > Mail. Under Send messages, check “Send immediately when connected.” Set the “Send/Receive” group frequency to 5 minutes or longer. This prevents Outlook from constantly polling the server for new mail, which reduces network and CPU overhead.
6. Apply Registry Key for Online Mode Enforcement
   Open Registry Editor. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode. Create a DWORD value named EnableCachedMode and set it to 0. If the Cached Mode key does not exist, create it. This ensures that even if a user profile is recreated, Outlook stays in online mode. Restart Outlook.

Additional Performance Tuning for Citrix Sessions

1. Disable RSS Feeds and SharePoint Folders
   Click File > Options > Advanced. Under RSS Feeds, uncheck “Sync RSS Feeds to the Common Feed List.” Under SharePoint, uncheck “Sync SharePoint lists and libraries to Outlook.” These features add unnecessary data synchronization that drains session resources.
2. Limit Send/Receive Groups
   Click Send/Receive > Send/Receive Groups > Define Send/Receive Groups. Select the group and click Edit. Uncheck “Download shared folders” and “Download folders subscribed to.” Set the schedule to “Schedule an automatic send/receive every 10 minutes.” This reduces the frequency of server requests.
3. Use Group Policy for Bulk Deployment
   In Group Policy Management Editor, navigate to User Configuration > Policies > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Other. Enable the policy “Do not use Cached Exchange Mode.” Also enable “Turn off all Outlook add-ins” under Add-ins. Apply the policy to the OU containing Citrix users. This ensures all published app sessions start with optimized settings.

If Outlook Still Has Issues After Optimization

Outlook Runs Out of Memory After Several Hours

Even with Cached Exchange Mode off, Outlook can accumulate memory due to unresolved add-in processes or large shared mailboxes. Set a session timeout in Citrix to force Outlook to restart periodically. In Citrix Studio, configure the published app to disconnect after 8 hours of idle time. Users must reconnect and relaunch Outlook, which clears its memory footprint.

Outlook Displays “Trying to Connect” Frequently

This occurs when the network connection between the Citrix session and the Exchange server is unstable. Open Outlook, click File > Options > Advanced, and under Send/Receive, set “Send/Receive” to work offline when the connection is slow. Alternatively, increase the connection timeout in the registry: navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\RPC, create a DWORD named ConnectTimeout, and set it to 60000 (60 seconds). Restart Outlook.

Outlook Crashes When Opening Attachments

This is often caused by antivirus software scanning attachments in the Citrix session. Configure the antivirus to exclude Outlook temporary files and the Outlook process (OUTLOOK.EXE) from real-time scanning. In Windows Defender, add exclusions for %USERPROFILE%\AppData\Local\Microsoft\Windows\INetCache and %USERPROFILE%\AppData\Local\Temp.

Item	Optimized for Long Sessions	Default Setting
Exchange connection mode	Online mode (no OST file)	Cached Exchange Mode with OST
Search indexing	Disabled for Outlook	Enabled and constantly updating
Add-ins	Only essential add-ins enabled	All installed add-ins enabled
Send/Receive frequency	Every 5–10 minutes	Every 1 minute
Hardware acceleration	Disabled	Enabled

After applying these optimizations, Outlook will run with lower memory and CPU usage during long Citrix published app sessions. The trade-off is that users lose offline access and local search speed. Test the configuration with a pilot group before deploying to all users. For further refinement, monitor Outlook’s memory usage with Citrix Director and adjust the Send/Receive interval based on your server’s capacity.