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.

This device doesn't meet the security requirements set by your email administrator

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.

Your account settings are out of date

How do I fix Outlook not updating emails?

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 Office 365 support guide 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

If that doesn't solve your issue, try the solutions below.An app password is used instead of your normal account password for apps that don't support two-factor authentication.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.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.

When using Microsoft Teams on an iPhone, slow performance and app crashes can bring mobile work to a halt. Many issues come down to internet bandwidth or low storage space. If there's a feature you think is missing from the Mail and Calendar apps, or if a feature in isn't working properly, you can provide feedback directly to the Mail and Calendar team.

Emails not syncing until Outlook restart

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.

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.