This is a big frustration for a lot of Odoo users: instead of fixing the issues with their Outlook synchronization, the Odoo team just writes a remark in their documentation to explain how the situation is. 

The conclusion is that is completely impossible to use the Outlook Agenda synchronization without sending spam mails for past events to all invitees...

It's clear that this is not usable at all and it seems that nobody at Odoo cares about it...