I was facing the identical problem and followed the (very clearly spelled out) steps in Vinod's reply, however this then created a different error:
Unable to read data from the transport connection: net_io_connectionclosed
I did a bit more digging and poking around and (while I'm not sure why this worked) I solved it by:
- Going back into IIS6.0 management console
- Open SMTP Virtual Server properties
- On General tab, changing the 'IP Address:' setting back to '(All Unassigned)'
Not sure why this works, but hopefully will help out someone facing the same problem in the future.