Question : 'Unknown Error 0x800C8100'

OL 2003/Exchange 2000
Whenever  I try to reply to an email offline, it hangs in my outbox and I get the following Send/Receive Error:
Task 'Microsoft Exchange Server - Sending' reported error (0x800C8100) : 'Unknown Error 0x800C8100'
I did detect and repair also set up a new profile, and created a new OST.  Still gets the same error when working offline.  No problem sending mails when I am working online.  
Can you help me to get out of this

Answer : 'Unknown Error 0x800C8100'

Try unchecking that box, then restart Outlook and see if you can reproduce the problem.