Outlook is configured to run in Cached Exchange Mode – this means that a copy of your mailbox is store on the local drive of your computer. When you use Outlook on a workstation for the first time it will download this copy of your mailbox in the background. Depending on the size of your mailbox this can take some time and Outlook will run slower during this process. If you open a folder and it appears empty, check the bottom right hand corner of the Outlook window and you will see that Outlook is downloading the items from the email server.
Once your mailbox has finished caching a full copy will be store on your workstation drive (there is also a live copy on the email server). Every time you close and restart outlook it will contact the email server and check for new mail. This process compares the mailbox on the email server with the copy on your C drive and downloads any changes – depending on the size of your mailbox, the number of items in your mailbox and the amount of time since you last had Outlook open, this process can take some time. In addition to your primary mailbox any additional mailboxes you have open in Outlook are also cached to your machine.
The cached mailboxes are stored in what is known as an OST files (Offline Data File). The size of your OST files depends on the size of the mailboxes and if you have a large OST file this can experiences performance problems within Outlook. The absolute maximum recommended size is 50GB but however Microsoft have issued the below guidance: highlight issues experiences as the size of your OST increases:
Up to 5 gigabytes (GB): This file size should provide a good user experience on most hardware.
Between 5 and 10 GB: This file size is typically hardware dependent. Therefore, if you have a fast hard disk and lots of RAM, your experience will be better. However, slower hard disk drives, such as drives that are typically found on portable computers or early-generation solid-state drives (SSDs), experience some application pauses when the drives respond.
More than 10 GB: When the .ost file reaches this size, short pauses begin to occur on most hardware.
Very large (25 GB or larger): An .ost file of this size increases the frequency of short pauses, especially while you are downloading new email messages.