Fortunately, Exchange/Outlook is smart enough to only download the changes to the OAB instead of the entire thing every day.There are still some circumstances where the entire OAB will be downloaded again, which makes it very important to understand the size of your OAB so you know just how much of your networks bandwidth will be used when all Outlook Cached Mode clients perform a full download (reference ).
This issue may be caused by a Microsoft Online server issue.
Sign in to the Microsoft Office 365 portal with your administrator credentials, and then click Service Health on the Admin Home page to determine whether other customers are experiencing the same issue.
For Users where Outlook is in Cache mode, They got to have their Outlook Address book Updated ,when they can access the latest Address book while they go offline.
If Outlook is left running constantly in Cached Exchange Mode, it updates the Offline Address Book automatically about once a day, depending on Address Book updates on the server running Exchange.
This file is also known as the offline data file which can be recognized easily when looking at the extension which is OST.
By default the file is stored in The advantage of Cached Exchange Mode is that mailbox content is also available when you don’t have a connection to Exchange, excluded the new items which haven’t been cached.
There is no GUI management tool of offline address book as of now.
Here, I will show steps to configure offline address book in Exchange 2013.
Issues can occur when the OAB for an organization grows to a large size, sometimes in the hundreds of MB.
Things that contribute to this size are things like number of recipients in AD, number of distribution groups, populated user attributes, & certificate usage (reference ).
It’s important to note that GAL Photos are NOT stored in the OAB.