NOTE – this process can take anywhere from 30 minutes to 2 hours to complete (depending on the size of your Mailbox), so please allow enough time.

Microsoft Outlook stores data (E-mail, Calendar items, Contacts, etc.) in a file.  If you use Outlook at home and like many users are not connected to an Exchange server, the file is called a PST (Personal Storage) file and is stored on your computer.  If you are faculty or staff at the iSchool or some other business and do connect to an Exchange server, the file is called an OST (Offline Storage) file and contains copies of all the data which is actually stored on the server.

Regardless of what type of file you have, the size of this file is limited if you want good performance and reliability. Although the absolute maximum size of OST files keeps growing (at the UW the limit is currently 100GB), for optimal performance we recommend you keep your OST file much smaller, less than 10GB is ideal

These instructions will explain some best practices so that your mail store (read: Mailbox size) never grows too much and shows you how to determine the size of your mail store.

General Best Practices

• Regularly empty your Junk E-Mail and Deleted Items folders.
• Don’t save attachments in Outlook. Instead, save the files to your cloud storage, your computer (or some other media), and then delete the message.  At the iSchool your computer is backed up so all the files saved locally will be safe.
• Do not send messages to yourself.
• Do not send large (read: bigger than a couple of MB) attachments via email, instead put the file in the cloud and send a link to the location so the recipient can view or download it.
• If you do forward a message with a large attachment, delete the item from your Sent Items. There is no need to keep the file in your Inbox and your Sent Items folder.

Outlook Microsoft 365

1) Check to see how big your mail store is.

– In the left pane of Outlook, right-click your account and select Data File Properties.

– Under the General tab click Folder Size.

2) Sort mail by size.

– In the left pane of Outlook expand Search Folders.

– Click on Large Mail to activate it. By default, your large mail items will be sorted by size with the largest on top. Since large items always contain attachments now you can easily save the files and delete the messages.

3) Using the Mailbox Cleanup tool.

– Click the File menu > Tools > Mailbox Cleanup

– We recommend all the options in the Mailbox Cleanup tool except AutoArchive. We do not ever recommend archiving your email and instead always recommend keeping all of it in your Exchange Mailbox.

– The Cleanup tool can remove redundant messages in any folder or any conversation. Cleaning up redundant items in conversations is a great way to free up space. Here is a video demo of the Cleanup tool, the “Clean Up Conversation” feature is mentioned at 5:12:

Outlook on the web

1) Check to see how big your mail store is.

– Click the gear icon (Settings) > View all Outlook settings > General > Storage

2) Clean up your Mailbox.

– While in Storage, empty your Deleted Items folder.

3) Apply Retention Policies.

– Right click every top level/parent folder and select Assign policy. We recommend choosing 6 Month or 1 Year.