Microsoft Outlook offers two connectivity modes — Online Mode and Cached Exchange Mode. While Cached Exchange Mode is the default mode for Exchange, Outlook.com, and Microsoft 365 accounts, users can switch to online mode anytime.

But what’s the difference between the Online Mode and Cached Exchange Mode in Outlook?

That’s exactly what we are going to discuss in this article. This article will also help you understand how these Outlook connectivity modes work, their advantages, disadvantages, and which one to choose while configuring your email account in Outlook.

The core difference between Cached Exchange Mode and Online Mode is that the former allows users to seamlessly access their mailboxes even when the network is poor (high latency), or the system is offline, while the latter needs constant and reliable network connection to work.

Below we have discussed both Outlook connectivity modes in detail, and their advantages and disadvantages.

Cached Exchange Mode

Cached Exchange Mode, also referred to as Offline Mode, allows Outlook users to access their mailbox items even when the system is offline. It was introduced with Outlook 2003 and is widely used by organizations.

When Outlook is configured in Cached Exchange Mode, it creates an OST file to store a local synchronized copy of mailbox items, such as emails, contacts, calendar items, notes, tasks, etc. It also downloads and stores the Online Address Book (OAB) to provide quick access to your contacts.

Outlook stores the changes made, such as an email draft, add or remove rules, contacts, etc., in the OST file and frequently synchronizes the mail items and OAB with the mailbox on the server.

Note: Outlook automatically checks for the mailbox updates frequently and OAB updates daily when network connection is fast. You cannot change the frequency as it is automated and depends on the network connection. However, you can ensure frequent synchronization by using faster internet connection.

Thus, when the network is poor, unreliable, or unavailable, users don’t notice any performance lag or encounter connectivity errors. The changes done in Outlook are stored locally and synchronized to the mailbox server as soon as the connection is restored.

Offline Storage Table or OST file, also referred to as Offline Folder, is an Outlook data file that stores a local cache of mailbox data. The maximum OST file size depends on the Outlook version you are using,

For instance, Outlook 2007 and earlier versions support an OST file with size up to 20 GB (UNICODE). However, Outlook 2010 and later versions support an OST file with maximum 50 GB size (UNICODE). Although the OST file size can grow larger than the recommended 50 GB limit, it can lead to synchronization and performance issues in Outlook. Large or oversized OST files are also prone to corruption that can lead to data loss.

Advantages

Disadvantages

Tip: You can use an OST to PST Converter tool to view, convert, and save mail items from an orphaned or inaccessible OST file to an Outlook importable PST format.

Online Mode

Online Mode is recommended for users who do not have enough storage to store a local offline copy of mailbox items, i.e., OST. The mode is preferred on devices with either limited storage capacity, such as tablets and ultrabooks, or a non-persistent VDI (Virtual Disk Image) environment.

When you configure an email profile in Online Mode or switch your Exchange or Office 365 profile to Online Mode, Outlook connects directly to the mailbox server. It maintains a constant connection with the mailbox server at all times. Thus, it requires a stable and low-latency network connection to work properly.

If the mailbox server goes down due to maintenance tasks or connectivity between the Outlook and mailbox server breaks due to poor or inconsistent network connection, Outlook becomes unusable and prevents users from accessing their email account and mailbox items until the connection is restored.

You can switch to Online Mode while setting up an Exchange or Office 365 profile or later after the setup is done. Also, Cached Exchange Mode is not available for IMAP and POP-based email accounts. These accounts are set up in Online Mode only.

Important Note: Online Mode option is not available for the Outlook.com account.

Advantages

Disadvantages

Tip: You can easily switch between Work Online and Work Offline Mode in Outlook. Go to ‘Send/Receive’ tab and click ‘Work Offline’ to go offline.

Following table summarizes the differences between Cached Exchange Mode and Online Mode.

Both Outlook connectivity modes are available in Outlook 2003, 2007, 2010, 2013, 2016, and 2019.

Recommended Read: How to Turn Off Cached Exchange Mode in Outlook

Microsoft recommends the default Cached Exchange Mode for Office 365 and Exchange Server accounts for a better and seamless user experience. Cached Exchange Mode ensures consistent Outlook performance in both low and high latency networks . Users can choose how much data they want to keep offline in OST file. This varies from 3 days to all data (may vary based on Group Policy applied by your organization). It also reduces network and traffic load on the Exchange Server.

However, if you use devices, such as Ultrabook or Windows tablets, with limited storage space, you should configure Outlook in Online Mode. You may also use Online Mode if you want to access Shared Mailboxes as they can significantly increase the local cache size (OST file size) that can cause performance lags in Outlook and may damage the OST file due to oversizing.

Sometimes, switching from Cached Exchange Mode to Online Mode may also help fix several Outlook issues, such as problems or delays in sending or receiving messages, outdated mailbox or calendar items, synchronization issues between the OST and mailbox server, etc.

These issues usually arise due to problems with Outlook data file or OST. However, you can fix this by repairing the OST file using the Inbox Repair Tool (SCANPST.EXE) or replace the damaged OST file by recreating a new OST file.