Optimizing Outlook for large mailboxes

Note: this article applies to anyone employing an Exchange mailbox, including those who subscribe to Office 365, AppRiver, or any hosted or in-house Exchange service. It does not apply to IMAP or POP mailboxes. If you are not using an Exchange service, you should be. We would be happy to assist!

If you find Outlook is running slow on your computer, it may be due to a large mailbox. Outlook works best when the mailbox size is under 2 GB. If you’ve been storing mail for a lot of years, and if many of the old messages have attachments, you can rack up a pretty hefty mailbox size before you know it. By the way, besides your email, your mailbox also contains your calendar and contacts.

Find your mailbox size

It’s easy to find the size of your mailbox. The instructions below are for Outlook 2016, but here are instructions for earlier versions as well.

  1. Right-click on the top item in the navigation panel in Outlook 2016 and choose Data File Properties.

  2. Click Folder Size

  3. Look for the Total size (including subfolders) This size is in kilobytes. In order compare this to the 2GB we were talking about, you will need to covert the number to gigabytes. The easiest way to do that is to divide the kilobytes by 1000000 (that’s 1 followed by 6 zeros), or you could use an online converter like this one. The 1000000 division won’t be as accurate as an online converter, but it will get you close enough.

Exchange server basics

You may have never given much thought to how mail gets synced to all your devices, but a simple understanding of how the system works will help you appreciate our suggestion for speeding up your large mailbox.

If you’ve ever rented a post office box, you already know the basics of how an Exchange server works. In the post office scenario, all your mail gets delivered to the post office, where someone sticks it in your mailbox. You have to go to the post office and open your mailbox to receive your mail.

An Exchange server is to email what the post office is to regular mail. All email addressed to you gets delivered to the Exchange server where it is then placed in your email mailbox. When you start Outlook, Outlook goes online and “picks up” the mail from the server and displays it on your computer.

The big difference between the post office and an Exchange mailbox is that when you “pick up” the mail from the Exchange server, you are only getting a copy of the messages. The original messages stay on the server where they can also be delivered to your phone and mobile devices. Another difference is, if you delete a message from Outlook (or phone or tablet), that message is then deleted from the server.

Controlling mailbox size in Outlook 2013 and 2016 for Windows

If you receive mail on your phone or tablet, you may know that these devices typically only store a few days or weeks of messages. There is a setting on devices that allows you to specify how many days or weeks of mail to store. This keeps old messages from eating up your valuable storage [A1] .

Well, as it turns out, there is a similar setting in Outlook 2013 and 2016 for Windows that allows you to specify how much mail to store. Mail that “drops off” your computer after the specified storage limit is reached can still be retrieved from the Exchange server at any time. This storage setting is called Cached (pronounced cashed) Exchange Mode. In the picture below, I can specify the number of weeks or months of mail I wish to store on my computer by dragging the slider left or right. In the picture I am storing 12 months of mail.

Here is how to find this setting.

  1. In Outlook Ribbon, click File > Account Settings > Account Settings.

  2. In the Email Accounts dialog box, double-click the name of your Exchange account. This should take you to the settings you see in the picture above.

Turning off Cached Exchange Mode

You may have noticed the check box next to Use Cached Exchange Mode. Unchecking this option might appeal to those who don’t want to store any mail on their PC, essentially reducing their Outlook mailbox size to zero. You would still see all your mail in Outlook, but you would be looking at it as it is streamed to your computer from the server in real time. Before you get too excited about this – and it is pretty cool – keep in mind you must have a decent Internet connection for this to work. So, if you work wirelessly from a laptop, access to your email will be affected by your Internet connection. No Internet, no email. Weak Internet, slow email.

Finding old messages

If you have reduced the size of your mailbox by reducing the number of weeks/ months of cached messages, you can still access mail that is no longer stored on your computer. Using Outlook’s search feature, Outlook will offer to find more messages on the server by providing a link for you as part of your search results.

We recommend

If your mailbox is under 2GB, we recommend leaving everything as it is, but if you are over 2GB you may have a better Outlook experience if you change the Cached Exchange Mode settings to 12 months, or less if you don’t need to see messages that old.

[A1]Only on outlook 2013 and beyond