So, in case that this is your scenario, the only advice that I can offer is: Be patient and read the information 🙂 By default, Outlook client is configured with the option of cache mode.
The meaning of “cache mode” is that Outlook client, will try to download all the Mailboxes content from the Exchange server to the local hard drive and save the information in a local Database (OST File).
Using a local copy of the Mailbox content should significantly improve the Outlook performance (response time) because access to the “data” (Mail items) created by using a local copy of the Mailbox that stored on the local drive.
Instead of “fetching” the mail from the remote Mailbox, each time a user click on the mail item.
One of our users complains that he cannot synchronize mail from his Mailbox in the cloud (Exchange Online).
I know that this article could consider as “long and tedious” but the Murphy law dictates that: most of the Outlook synchronization problem accrues to “VIP Users,” that usually demand a solution!
For example, we create a new Mailbox (empty Mailbox) in Exchange Online, and we want to copy the data (mail items) from a PST file to the Exchange Online Mailbox.
When using Exchange Online services, the communication channel is based on a WAN infrastructure, which by default, have narrow bandwidth verse LAN infrastructure.
Before we begin the troubleshooting process, we should classify the Mailbox as Standard Mailbox or “Large Mailbox”.
When using the term “Exchange Mailbox”, we are using an “alias” for a Database that hosted on the Exchange server.We can describe the OWA (Outlook Web Application) mail client as a “viewer” for the data that stored in the Exchange database, and the Outlook mail client as a “viewer” for the database (OST File) that stored on the user desktop (Hard drive). WAN environment: When using a local Exchange Server (on-premises), Outlook synchronizes data to and from the Exchange Mailbox, by using a local LAN infrastructure.