Microsoft often blocks Nylas from connecting properly after authentication and the only items synchronized have the subjects "Your email access has been blocked" or "Microsoft account unusual sign-in activity". This also prevents sending with the "Message Delivery Submission Failed" error.
This occurs because the accounts are using Microsoft Basic authentication which Nylas calls "Exchange" or "Outlook" and the accounts don't have 2FA enabled. Microsoft restricts third party apps from being able to connect to these accounts until the users turn on 2FA.
Please have the user activate two factor authentication by following the link here: https://support.microsoft.com/en-us/account-billing/turning-two-step-verification-on-or-off-for-your-microsoft-account-b1a56fc2-caf3-a5a1-f7e3-4309e99987ca
Then have the user generate an App Password by following the instructions on this link: https://support.microsoft.com/en-us/account-billing/using-app-passwords-with-apps-that-don-t-support-two-step-verification-5896ed9b-4263-e681-128a-a6f2979a7944
Have them authenticate with Nylas again and use that App Password in place of their normal password. This normally fixes the Nylas account.
If the use can't turn on 2FA or generate App Passwords, please have them reach out to their email administrator to get that enabled.