As Exchange administrators we tend to think about servers, networks, and datacenters. While all that is important, the end-users think of email as the client experience. In other words, is Outlook running nicely, or is it running slowly?

One of the biggest factors that influences Outlook performance is caching.

What is Outlook Cached Mode?

When you connect Outlook to an Exchange account, Outlook downloads a copy of your mailbox content to a local cache. This is known as “cached mode”.

Outlook caching provides two benefits. First, it improves performance. Outlook performance in “on-line mode” is not all that great. In the old days, when Outlook was primarily used to connect to Exchange servers that were close to the end user, on-line mode was good enough. These days, Exchange mailboxes are hosted on servers on centralized datacenters, or in the cloud in Office 365. Users connect to their mailbox over longer distances. They use Outlook over slow networks such as home internet connections and public Wi-Fi.

Second, Outlook cached mode allows users to work with mailbox data while offline. Users can read and compose emails without network connectivity, such as when traveling. When they reconnect to a network, their emails are sent, and any new emails are downloaded for them to read.

What is an OST File?

Outlook’s local cache of mailbox data is stored in a file format known as an OST file. OST files are Outlook offline data files.

The OST file format has a maximum file size that depends on the version of Outlook you’re running:

  • Outlook 2010 and later support an OST file size of up to fifty gigabytes (50GB)
  • Outlook 2007 and 2003 and earlier support an OST file size of up to twenty gigabytes (20GB)
Hopefully you’re not running Outlook 2007 or 2003, both of which are unsupported today. Outlook 2010 is also out of mainstream support, so hopefully if you are still running Outlook 2010 you are planning an upgrade in the near future.

The limits above apply to OST files in Unicode format, also known as “Large format”. The older ANSI format has an OST file size limit of only 2GB. Today, Outlook will use the Unicode file format by default.

With the huge size of mailboxes possible in Exchange Online and Exchange on-premises, the size limit of OST files is a problem. Can an OST file hold all the data in a 100GB mailbox? No, it can’t. In fact, if the Outlook cache grows to the maximum file size, the user will begin to see problems. New mail won’t appear in Outlook, and they will be unable to move existing mail between folders.

Large OST files are also prone to corruption. Recovering from OST file corruption means deleting the file, and then allowing Outlook to download the mailbox contents to a new OST file. Rebuilding the OST file for a large mailbox can take a long time, and the user has a degraded experience while the rebuild is occurring.

But there are solutions so that you can avoid those issues.

How to Manage OST File Sizes Using the Outlook Cached Mode Slider

With the large mailbox quotas for Exchange Online mailboxes, Microsoft thought it prudent to implement a method for partial caching of mailboxes. With Outlook 2013 they introduced the cached mode slider, also sometimes referred to as the OST slider. Before this feature was developed, caching a mailbox in Outlook was an all or nothing proposition. The OST slider allows you to control how much of your mailbox data is cached to your computer’s hard drive.

In Outlook, go to File -> Account Settings, then highlight the account and click the Change button. There you’ll see the slider, which is shown in the screenshot below.

OST file size managed by Outlook cached mode slider
The Outlook cached mode slider, managed by Group Policy

The Outlook cached mode slider does not directly control the size of your OST file in gigabytes. Instead it allows you to select how much data is downloaded from your mailbox by age. The slider’s default setting depends on size of the hard disk in your computer.

  • Hard drives up to 32Gb in size, default to 1 month
  • Hard drives bigger than 32GB, but less than 64 GB, default to 3 months
  • Hard drives 64GB and larger, default to 12 months

The slider can be configured using Group Policy. You will need to download the Group Policy administrative templates for your version of Office. The Office 2016 administrative templates are available here.

OST file size management using Group Policy
Group Policy settings for managing the Outlook cached mode slider

If Group Policy is not available, you can use the registry to configure the Outlook cached mode slider. The registry key used is HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\xx.0\Outlook\Cached Mode. Replace “xx.0” with the appropriate number for your version of Outlook:

  • Outlook 2016 is 16.0
  • Outlook 2013 is 15.0
  • Outlook 2010 is 14.0
  • Outlook 2007 is 12.0
  • Outlook 2003 is 11.0

Create a DWORD value of SyncWindowSetting, and then set the desired value:

  • 0 = All (whole mailbox)
  • 1 = 1 month of email items
  • 3 = 3 months of email items
  • 6 = 6 months of email items
  • 12 = 1 year of email items
  • 24 = 2 years of email items
  • 36 = 3 years of email items (Outlook 2016 only)
  • 60 = 5 years of email items (Outlook 2016 only)

For Outlook 2016 there is an additional range of settings that can be applied using a DWORD value of SyncWindowSettingDays.

  • 3 = 3 days
  • 7 = 7 days
  • 14 = 14 days

For Outlook 2016, both registry values must be configured together. Configure the desired setting on the appropriate registry value, and set the other value to 0. For example:

  • To set the slider to 3 days, configure SyncWindowSetting to 0, and SyncWindowSettingDays to 3.
  • To set the slider to 6 months, configure SyncWindowSetting to 6, and SyncWindowSettingDays to 0.
  • To set the slider to “All”, configure SyncWindowSetting to 0, and SyncWindowSettingDays to 0.

How to Manage OST File Sizes Using the Registry

You can also use the registry to control OST file size limits. Microsoft has documented the registry keys, entries and values here. There are two registry entries for setting the maximum size that an OST file can grow to:

  • MaxLargeFileSize – to set the Unicode format OST file size limit, up to a maximum of 50GB for Outlook 2010 and later, or 20GB for Outlook 2007 and 2003.
  • MaxFileSize – to set the ANSI format OST file size limit, up to a maximum of 2GB.

Why Manage OST File Sizes at All?

The most important question I can answer here is why both worrying about OST file size at all. Here’s some examples of why this may be necessary.

  • Roaming users may need their entire mailbox download to their laptops for use in offline situations such as airplanes.
  • Hotdesking employees, or multiple employees who use the same computer, may mean that there isn’t space on workstations for multiple employees’ OST files.
  • Users who open multiple shared mailboxes likewise may not have space on their hard drives to download offline copies of all the data from those mailboxes.
  • Virtual Desktop Infrastructure (VDI) often requires that users run Outlook in online mode, with no local OST file at all, resulting in poor performance for the end user. If storing OST files in terminal server and VDI environments is not desirable, I’d recommend using Outlook web app (OWA) instead.
  • Tablets with small hard drives may not have the space for a 50GB OST file.

In any of the above cases, you may find it necessary to use Group Policy or other management tools to configure different Outlook cached mode sliders, or different OST maximum file sizes.


Abdulsalam Garba

The author Abdulsalam Garba

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