When disabling Skype for Business Online, sometimes a long string of characters appears in front of the SIP Address. This is then synced to all your contacts in Outlook & Skype for Business.
Such as: firstname.lastname@example.org
To resolve this, navigate to the Office 365 portal and open the "Exchange Management Page". Open a user account and navigate to "Email Addresses". Remove the long string from the front of the "SIP Address" proxy field.