Server Side Synchronisation Not Working

Hi there,

Since last Friday, appointments are not being synced between the CRM and Microsoft Outlook calendars for users. I have retested and enabled mailboxes, and also applied default settings and it seems to show as being successful for all mailboxes.

I have been using CRM for 4 years and I have had this issue before but within a day or two it resolves itself. However it's been 5 days. 

No updates or customizations were made during this period. 

Any ideas?

Thanks