If you were not previously set up to use the "Cached Exchange Mode" with your Outlook client, then when you switch your mail server from an in-house Exchange server to Outlook 365, you may feel that your Outlook client is slower and sometimes even times out.

It is a way that Outlook caches your mailbox on your local drive and lets Outlook synchronize with the mail server periodically on a batch basis.  This is especially helpful if you use a notebook computer at the office and you carry it with you to home or on business trips.  You can continue to use Outlook even if you don't have an Internet connection.  Then, as soon as the Internet is restored, your notebook computer will synchronize with the mails on the Exchange servers, and the pending emails on your local notebook will be sent.

The disadvantage in using Cached Exchange Mode is that it functions in somewhat less than real time.  Therefore, if you are using a desktop computer in-house, you may want to consider not using Cached Exchange Mode.  You will send and receive your emails in real time and you may feel that it is more responsive.

When your organization switches from an Exchange server to a hosted solution like Outlook or Office 365, your mail server no longer resides locally.  If you do not use the Cached Exchange Mode, you may feel that your Outlook client is slow and sometimes even times out.  This is because the traffic between your Outlook and the mail server is no longer on the local LAN.  They have to go through the Internet, which will have a longer round-trip time and may even increase the possibility of lost packets.  Configuring your Outlook to use the Cached Exchange Mode will help to resolve this problem and make your Outlook experience much more pleasant.

The following is an example using Outlook 2010.  Other Outlook versions will have a similar user interface.  First, you need to change your account settings in Outlook. With Outlook 2010, you can go to "File" and choose "Info," then "Account Settings." The following window will show up:

Highlight the email account, and click on "Change...," after which the following window will show up:

Click on the checkbox next to "Use Cached Exchange Mode" and save it.  Exit Outlook and come back again.  The first time you start up Outlook, it may take a little while because it has to download the mailbox information to the local drive.  Once this process is done, the future operation of your Outlook client will be smoother when using Outlook or Office 365 as the hosting mail server.