This error could come in various scenarios. Mine was that I’ve changed the primary address of all users on an Office 365 tenant. After this change some Outlook have this errors, the majority of them have a shared mailbox configured.
To solve it, simply clear the auto-complete list :
Outlook 2010-2013 :
- Open File Tab.
- Click on Options.
- Click on Mail (Posta).
- Under Send Messages (Invio Messaggi), click on Empty Auto-Complete List (Svuota elenco di completamento automatico)
- Close and Open Outlook.
Outlook 2007 :
- Close Outlook
- Open File Explorer and digit %APPDATA%\Microsoft\Outlook .
- Delete the file Outlook.nk2 on this folder (can be more than one, in base of how many profile are configured on the pc)
- Reopen Outlook.