IT Administrators are familiar with the complaints. “Outlook seems sluggish.” “Why does it take so long to sort items in my inbox?”
If these complaints from your users sound familiar, the culprit may be too many items in a folder.
For users running Microsoft Office Outlook in cached mode, the problem generally manifests itself in sluggishness of the client, as Outlook sifts through large amounts of data, resulting in a dissatisfactory e-mail experience for users.
So how many items are too many items in a folder? Really, from an Exchange best practice point of view, you don’t want to see more than 20,000 messages in a single folder. And that is particularly true for a critical path folder, including the Inbox, Contacts, Deleted Items, Calendar, or Sent Items folders.
The immediate question, what steps can a user with a large number of items in their mailbox take to reduce his item count? The easiest is solution is to enable Auto Archive in Outlook.
As an IT Administrator, you can encourage your users to enable Auto Archiving. The Microsoft Support Knowledgebase provides an article with instructions for setting up Auto Archiving in Outlook 2010, 2007, and 2003.
You also might want to implement a mail retention policy for your organization, which will auto delete items that are older than the limit in the policy. You can open a support request with Microsoft Online Services Technical Support to set up a retention policy for your organization.