Office 365 email sync/update issues

McDeth 

That's a new one on me, but the old support rule of divide and conquer should serve here as well as it usually does.

Things I would try;

* Rule in/out server-side issues = Have the user makes some changes on their phone and see if those are married up to their Office365 mailbox.

* Rule in/out Outlook/Office = Configure the users mailbox using the in-built Mail app in Windows 10. Check and see if the issue is reproducible. If, like me, you hate the in-built mail app, try Thunderbird instead.

* Rule in/out the mailbox = If the user is has access to a shared mailbox, make changes to that, and see if those are married up to Office365.

*Rule in/out the workstation itself = Recreate the users Outlook profile within another user profile on the same and/or a different workstation to see if it's reproducible.

Gut instinct is it's an upload issue, but those would typically manifest far more dramatically than what you're seeing. Also try Ctrl+Rightclick'ing the Outlook icon in the notification area. The "Connection Status" is usually typically fairly useless but it may shed some light in this situation.

Lastly, check there are no updates pending for Office itself.