Email is crucial to most businesses and Outlook is one of the most popular mail clients so it's hardly surprising that we get a fairly large amount of support logs for Outlook related issues.
One thing that can effect Outlook performance is the sheer amount of mail items in the mailbox. While this has always been the case it's more relevant than ever these days as the volume of business emails being sent/received is only increasing.
More recent versions of Outlook have the handy feature of allowing users to open multiple Exchange/Office365 mailboxes in a single Outlook profile. This is great for things like Shared Mailboxes, or for keeping an eye on a colleagues mailbox while they're on holiday. Unfortunately having multiple mailboxes open will add to the folder count and can impact your performance.
Microsoft specify the following limits to avoid hitting:
Outlook 2007:
50,000 items per folder
500 folders total
Outlook 2016, Outlook 2013 and Outlook 2010:
100,000 items per folder
500 folders total
We have encountered these issues with Outlook running in both online and cached modes, lowering the folder/item count down below the limits has helped in both.
To avoid potential issues we would recommend users keep on top of maintaining their mailboxes, deleting items that aren't required (shift+delete will permanently delete those items you're certain aren't required), archiving items that are only required infrequently (remember to backup your archives!) and try to keep additional mailboxes to a necessary minimum.
You can get a count of items in a folder in Outlook by right clicking the folder, choosing properties and then changing from Show number of unread items to Show total number of items. Unfortunately this is a per folder setting and doesn't help with the folder count.
Thankfully we have found a handy VBA script that can be used in Excel to list your Outlook folders and how many items they contain. Please be aware it takes a small amount of VBA knowledge to use, if there's interest we'll follow up with some step by step instructions.
We hope you'll find this information useful if you come up against Outlook slowdowns. If you're having problems with your business email and would like some help click here for our contact information.
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Outlook