What is Online Mode?
Online Mode maintains a connection to the Exchange Server for all your mailbox access, this mode typically requires a stable connection to the server, as far as latency is concerned. Online Mode works well in environments where users have no persistent storage to which they can store their OST, such as VDI scenarios or on devices with limited storage capacity i.e. tablets.
What is Cached Exchange Mode?
As the name states, a copy of your mailbox is saved on your computer, this provides quick access to your data and is frequently updated with the server that runs Microsoft Exchange. By default, this mode is enabled whenever you add an Exchange or Office 365 Account.
Depending on how your organisation is set-up, you can change how much mail to keep offline/cached on your system, this can vary from as little as 3 days to ALL the data (since the account was created). The main purpose of this cache is to reduce network traffic, reduce the payload on the Exchange Server(s) and provide offline availability (in-case you ever have network troubles – this cache, or .OST file* contains all your cached content).
*What is stored on the .OST file?
Generally, your mailbox data includes mail, calendar, shared folders and additional mailboxes linked to your Exchange account.
Why does this matter?
Depending on which mode and settings are enabled, by changing the amount of data to synchronise can affect the time it takes Outlook to open as well as the amount of storage space Outlook uses on your computer. Outlook reads and writes items into the .OST and how well this responds usually depends on how many items are stored within the file & the speed of how this information is accessed.
So, which one is best for me?
For Office 365 or other hosted Exchange customers, the answer is easy – Cached Exchange Mode. Typically, latency might be higher and cached mode will accommodate this.
There are a few options which can be used to alleviate slowness with larger mailboxes, such as:
Reducing the total cache size in a range from 3 days to ALL.
Turning off automapping on your shared mailboxes.
Turning off Shared or Public Folders.