This article describes best practices on organizing large mailboxes, avoiding corruption of Outlook profiles and other collateral issues.

Which mailboxes are considered large?

Mailboxes that are getting close to 50Gb, with 500+ folders in profile, and/or that have more than 5000 items in one folder are considered large and exposed to corruption.

By default, Outlook creates a local copy of your mailbox data (OST file), which is especially helpful with slow internet connections. However, if this file becomes too large, this increases the chances that it might become corrupt. Microsoft even recommends keeping the OST file below 10GB as the speed of the hard disk is much lower than the amount of data that Exchange servers provide, and will reduce the efficiency of the Outlook-Exchange connection. Also, having more than 5000 items in one folder can lead to performance issues.

What are the symptoms of this issue?

Commonly, large mailboxes tend to slow down Outlook, break search engine indexing, and make Outlook client show the following error messages or behavior: 

  • Requesting data from Microsoft Exchange Server
  • Outlook is retrieving data from the Microsoft Exchange Server
  • Trying to connect
  • Outlook client is Not Responding

Note: the large mailbox issue can also affect mobile devices. In some cases, mobile mail applications won't synchronize messages, calendars, contacts, and other mailbox items. Some applications may not display any mailbox data.

How to check the size of a mailbox

To prevent the mailbox from growing too large, it’s recommended to check its size and the number of items in folders frequently. 

To check the size of the .ost (Outlook Data File), right-click the name of your mailbox in Outlook > Data File Properties > Folder Size

Note: the size of the mailbox is displayed in KB, so if your mailbox is larger than 50 GB it will be displayed as a larger number than 52428800 KB.

How to check the number of items in a mailbox

To check the number of items in folders, open Outlook and select the folder. The number of items in it will be indicated in the lower left corner of the Outlook client.   

Note: the number of items cannot be checked in OWA.

How to decrease the size of the OST file

Change the amount of cached emails

When new Outlook profile is created, it’s set by default to keep all mail offline. Since all your mail is stored on the server, it is not necessary to keep a copy of every message locally. To reduce the size of the OST file you can change the amount of emails that are cached from all time to 6 months or 1 month. This will leave all your emails on the server, but only the most recent ones will be downloaded into the OST file, which can help reduce the likelihood of it becoming corrupted.

To change the amount of mail kept offline, open Outlook, click Account settings > Account settings… > Select your email address > Change. Then you will be presented with the option to change the amount of mail to keep online.

Compact the OST file

To compact your OST file in Outlook, click File > Account Settings > Account Settings… > More Settings > Advanced > Outlook Data File Settings… > Compact Now

Compact OST tool doesn’t reduce the size of the OST file drastically, so it is better to use it in combination with other measures.

Outlook Archiving

Outlook Archiving allows you to store old items in a separate archive.pst file, making your profile lighter. Read the Knowledge base article on:

for more information.

Delete older items (Cleanup tools)

Mailbox Cleanup allows you to view your mailbox size, find and delete old items by size or by date, empty your Deleted Items folder and Conflicts folder.

To use Mailbox Cleanup, open Outlook client, click File > Cleanup Tools > Mailbox Cleanup

To find old items, select the number of days that passed from the moment the message was sent or delivered and click Find.

To find large items, define how large the message must be and click Find. Normally, the average size of a message is considered to be 75 KB, however, most items appear to be smaller.

After the search is complete, you can select and delete items that you don’t need. If you’d like to delete all items from the search results, select all items by pressing Ctrl+A on the keyboard and then pressing Delete.

Utilize Intermedia Email Archiving

To keep the size of your mailbox under control, you can use Intermedia Email Archiving. This service allows you to store all incoming and outgoing messages in a separate location from your mailboxes. This way you can delete old items in Outlook or OWA and still have access to them in the future. Additionally, there is no storage limit in the archive, so you can keep just the latest items in the mailbox, and be sure that all other emails are securely stored off-site, yet easily accessible with a single search.

Switch Outlook into Online mode

When new Outlook profile is created, it’s set by default to keep all mail offline. Alternatively, you can use Outlook in Online mode, which makes it instantly keep the connection to the server and view the data there. This way Outlook doesn’t store items locally on the computer, so the profile doesn’t get corrupted. However, Online mode can cause a decrease in Outlook client performance.

To enable Online mode:

  1. Open Outlook client, click File > Account Settings > Account Settings…
  2. Select your email address
  3. Click Change and uncheck Use Cached Exchange Mode.
  4. Click Next and restart Outlook client. 

Read the Knowledgebase article on What Is Cached Exchange Mode In Outlook for more information.

How to decrease the number of items in folders

Set up Outlook Rules to sort your items in folders

In order to sort your items in folders, you can create Outlook rules that will do it automatically. For example, you can create a folder with a certain senders name and move all items received from them into this folder, decreasing the number of items in the Inbox folder.

  1. Open Outlook client and right-click Inbox
  2. Click New Folder... Type in new folders name and press Enter.
  3. Select the message in Inbox by clicking it
  4. Right-click it, select Rules > Always Move Messages From *Sender Name*
  5. In Rules and Alerts select the folder you have just created. 

After you create the rule and click OK, the rule will be applied on the all messages from this sender, and those messages will be moved to the selected folder. 

Note: If there are many messages from this sender, this process may take some time.  

For more detailed explanations and opportunities of using Outlook Rules, check the article How Do I Create Outlook Rules Based On Specific Words?

For instructions on creating Rules in Outlook for Mac, check this Microsoft article.

Request sorting

In case creating Outlook rules or sorting items manually is inconvenient for you, you can contact Support and request to sort items from the back-end. Your items can be sorted in folders by year, quarter or month. This request is free of charge.