To further reduce the size of the ost-file and improve overall Outlook performance, Outlook won’t cache all your mail by default. This is also due to the fact that on most occasions, you no longer need direct access to all items.
By default, Outlook will cache up to 12 months of your mail if your disk is 128GB or larger. If your disk is between 64GB and 128GB, then it will cache up to 6 months by default. For even smaller disks (like for Outlook RT on a Surface tablet with 32GB), the account is automatically set to only 1 month.
When you are connected to your Exchange server, you’ll find the following remark and link at the bottom of your message list to access older items;
There are more items in this folder on the server Click here to view more on Microsoft Exchange
To change the caching period, to a smaller or larger value (all items is an option too!) use; File-> Account Settings-> Account Settings…-> double click on your Exchange account Here you’ll find a “Mail to keep offline” slider.
Note: There is a similar option for this for IMAP and Outlook.com (EAS) accounts but unlike for an Exchange account, there is no option to view the remaining server items in Outlook without modifying the cache slider. This is why the default for IMAP and EAS accounts is set to cache all mail unless your disk is smaller than 128GB.