I started getting a lot of errors in Office 365 out of nowhere recently. Since I am using EWS to access office 365, I have accounts with impersonating rights to make it work. Since yesterday, I am getting loads of “No mailbox with such guid” errors on valid mailboxes. Since EWS is considered as a third party, support cannot do anything about it. Accounts are licensed and registered. How do I go about to fix this issue myself?