This feature requires you to be using a Microsoft Exchange Server ,Office 365 account or outlook.com account
Turn on Cached Exchange Mode
On the Tools menu, click E-Mail Accounts, click View or change existing e-mail accounts, and then click Next.
In the Outlook processes e-mail for these accounts in the following order list, click the Exchange Server e-mail account, and then click Change.
Under Microsoft Exchange Server, select the Use Cached Exchange Mode check box.
Exit and restart Microsoft Outlook.
Turn off Cached Exchange Mode
On the Tools menu, click E-Mail Accounts, click View or change existing e-mail accounts, and then click Next.
In the Outlook processes e-mail for these accounts in the following order list, click the Exchange Server e-mail account, and then click Change.
Under Microsoft Exchange Server, clear the Use Cached Exchange Mode check box.
Exit and restart Outlook.