I have a new Windows 10 computer with Office 2013 on it and Outlook is now giving me this error message every time I try to Send and Receive.
Task '*** Email address is removed for privacy *** - Receiving' reported error (0x8004210E) : 'Your mailbox is temporarily unavailable because another e-mail message is being delivered to it or another mail application is accessing it. The server responded: -ERR [IN-USE] account is locked by another session or for maintenance, try again.'
I also use this email on my iPhone. Even if i turn off my iPhone i get the same error on my PC. Also Spectrum says it is not a problem on their end.