Regardless of their mailbox size, Microsoft set a default OST size for a reason, while it can be changed, that doesn’t mean it should. If your password isn't the problem, you might have a certificate error. If so, you'll see a message that says, "There is a problem with the server's security certificate. The security certificate is not from a trusted certifying authority." Check with your email provider for instructions about how to create an app password for your email account.

How to resolve the issue of Outlook not syncing in Windows?

In the notification bar at the top of the app, select Fix account. If your password is incorrect, you'll see a message asking you to update your password. Before you begin, make sure that your device has the latest operating system and app updates. Try the general troubleshooting solutions in Troubleshooting Mail and Calendar apps for Windows 10. If that doesn't solve your issue, try the solutions below.

Allow Mail and Calendar apps through Windows Firewall

How to force Outlook to sync manually?

In some cases, there might be outages or network-related issues, such as problems with a VPN. Mail flow issues can also arise out of other circumstances, such as a user having a mailbox in both Exchange Online and Exchange on-premises. In some cases, an issue might be related to MDM settings, such as restrictions on Outlook functionality, or user account and mailbox settings in Exchange. Sometimes an error appears in the Mail and Calendar apps when the apps try to sync new email or website calendar events. These syncing problems usually happen because of issues like a damaged Outlook profile, poor internet connection, or a corrupted OST file.

Check your security software

While we all use systems differently, I don’t think I have 100GB of data after working for my company almost 25 years, let alone this much email.If that doesn't solve your issue, try the solutions below.The approach that IT takes to address a syncing problem depends on what type of problem it is.The Mail and Calendar apps for Windows 10 don't support two-factor authentication.Firewalls and antivirus software might prevent you from syncing your email and calendar events.IT teams should build a plan to tackle these threats and make sure users are aware of the security concerns.Regardless of their mailbox size, Microsoft set a default OST size for a reason, while it can be changed, that doesn’t mean it should.Why not limit the caching to a more reasonable 1 year, if they need to access things outside of this, they can, they just have to click ‘view messages on server’ to do so.

Check the software documentation to learn how to temporarily turn it off. Firewalls and antivirus software might prevent you from syncing your email and calendar events. When issues occur at the account level, admins should ensure that users are properly logging into their accounts and provide them with instructions as needed. For example, admins might want to check the user's Exchange account settings and whether they can reach Exchange Server.

Check if two-factor authentication is turned on for your account

IT teams should build a plan to tackle these threats and make sure users are aware of the security concerns. If none of these steps work, the issue might be on the server side, which can make the troubleshooting process more complex. Admins must think about the nature of the problem and which users it's affecting to find the source. For example, ActiveSync might not be enabled, or there might be a problem with the ActiveSync certificate settings. In some cases, IT might be able to track down a problem by examining the Exchange logs.

Check the connectivity to your email server

Change the limit to 1 year, wait for the sync, check the size of the OST once Outlook confirms all sync has been completed. Now change the slider to 2 years and follow the same principal. Outlook has a hard limit of 50GB per OST, decrease the period of e-mails that get cached, delete the cache, and re-run the sync.