Outlook 2003 error 0x8004210E

I keep getting a error message when trying to access my orange email account “Task ‘Server_Name - 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 box locked” sometimes it is ok after the third or forth attempt but i do not have any problems if i turn off my Comodo internet security do you think Comodo internet security is the problem

Are you sure Outlook has the proper Firewall permission? Look under Firewall → Advanced → Network Security Policy → Application rules → Now look up Outlook and make sure it is either set to E Mail client or Trusted application.