By default, you can only synchronize one year of email in Legacy Outlook for Windows/Mac (and new Outlook for Windows). (In the new Outlook for Mac, automating syncing is enabled and takes place every 10 minutes). Syncing emails and other data is an easier way to manage data across multiple devices and ensure you are saving fewer duplicate files. 

Here's how to change your sync settings  in Legacy Outlook/New Outlook for Windows:

1. Open Outlook.
2. Go to File > Account settings > Account settings.

3. Select your email account and click Change. 
4. In the Offline Settings section, you will see an option to select "Use Cached Exchange Mode to an Outlook data file." Select this.
5. Below this, find the slide labeled "Download email for the past...."

5. Move the slider to "all" to download all emails offline or to a new time range. Select Next to complete the process. You may need to restart Outlook for the changes to be reflected.

NOTE: If you use an Exchange account, your information is automatically synched automatically, and manual adjustment may not be required.