My OST file is now 46GB and Outlook takes minutes to open. I deleted lots of old emails from my Exchange mailbox but the file size did not change. Why does it stay big and how do I make it smaller?

Deleted items leave empty space inside the OST. The file does not shrink until you compact it.

1. In Outlook go to File > Account Settings > Account Settings > Data Files.
2. Select your mailbox data file and click Settings.
3. On the Advanced tab choose Outlook Data File Settings, then Compact Now.

This can take a while on 46GB, so do it when you do not need Outlook.

A better long term fix is to cache less. In the account settings for your Exchange account move the Mail to keep offline slider to 6 or 12 months. Outlook then rebuilds a smaller OST and the older mail stays on the server.

Compact took about 40 minutes and it is now 31GB. Does Outlook have a size limit for OST files?

Yes. Outlook 2013 and later use a default limit of 50GB for both PST and OST files, and Outlook warns you at 95 percent of that. You can raise the limit with the MaxLargeFileSize and WarnLargeFileSize registry values under the Outlook policy key, but I would not. A file that big gets slow and the risk of corruption goes up. Use the slider to cache less instead.

Set the slider to 12 months. New OST is 9GB and Outlook opens in seconds. Thanks!