Fix Office 365 Email Sync

My Office 365 mailbox is no longer syncing across devices and I need a specialist to get it working smoothly again. I have already created the account, but something in the configuration is clearly off, as new messages appear on the web yet never reach Outlook on my laptop and phone.

Here is what I expect from you:
• Diagnose the root cause of the sync failure (cache corruption, OST/PST issues, autodiscover, etc.)
• Apply the appropriate fix in the Office 365 admin portal or local Outlook profile—whichever the issue dictates
• Test and confirm that mail now sends, receives, and syncs in real-time on at least two devices
• Provide a short note explaining the steps you took so I can reference it later

You may use whichever tools you prefer—Outlook desktop settings, Microsoft 365 Admin Center, PowerShell commands—so long as the problem is fully resolved. Once everything updates instantly on all devices, the job is done.