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.
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.
There is a similar option for this for IMAP accounts but unlike for an Exchange account, there is no option to view the remaining server items without modifying the cache slider. This is why the default for IMAP accounts is set to cache all mail.