How do I improve performance in Outlook for Windows?

This article will help you diagnose and resolve issues where users of Microsoft Office Outlook experience poor performance when working with Microsoft Exchange Server.

When such an issue occurs, Outlook users may receive one of the following messages:

In some cases, it means that you will need to troubleshoot connectivity issues. If you have already checked that Webmail (OWA) works fine, and noticed that there is an improvement in Outlook performance in Online Mode, it is likely that your mail client is having difficulties with working in Cached Exchange Mode. Usually, it happens to large mailboxes. These mailboxes involve large Outlook data files that are stored locally on the machine and are accessed every time Outlook tries to retrieve the mailbox data from the server. The larger this file is, the higher the chances that it might get corrupted, or might affect Outlook performance.

While the overall size of .ost files has a preconfigured limit of 50 gigabytes (GB), it is also possible to increase this limit. However, Microsoft recommends avoiding Outlook data files larger than 10 GB. Read the Microsoft KB article on the connection between Outlook data file size and Outlook behavior for more information. If you wish to increase the limit of your .ost file you may also refer to this article.

NOTE: Outlook is designed to be used as a mail client primarily and it is not designed to be used as a long term archive store of a large amount of email.

Even though the recommended number of items for newer versions of Exchange is quite high, having a large number of items in a folder means that operations in that folder will take longer, because most of the low-end computer hard drives are delivering less than ¼ of the performance that hosted Exchange servers can provide. In particular, Outlook performance is significantly influenced by the number of items in the most commonly used folders, such as Calendar, Contacts, Inbox, and Sent Items folder. 

The recommended number of items in a folder depends on several factors. These factors include the client's proximity to the server, the storage infrastructure, the load on the hard disks, the number of users, and the number of restricted views. 

The Microsoft-recommended limitations for the number of items per folder specify 100,000 messages per folder, 500 folders total.

NOTE: due to the nature of a shared environment, we do not recommend that you maintain more than 5,000 items in a folder.