There is a formal minimum requirement that we need to check when connecting to Office 365 Mailbox.
The supported versions are: You can read more information about the software requirements, in the article: Software requirements for Office 365 Enterprise and Office 365 Midsize Business The Office 365 desktop apps consider as crucial software component that need to be installed on each of the desktops that access Office 365 services (such as Exchange Online and Share Point Online).
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.
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”.
(We will review later some tools that can help us to disable or to locate these applications).
When dealing with “Outlook synchronization problems” there are two main scenarios: Many times we meet the synchronization problem when we need to upload data to the Exchange Online Mailbox.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).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.(The synchronization process “direction” is from the local desktop to the cloud).