i was searching google to try and find a solution to the problem and stumbled on this thread.

I've got exactly the same problem. OE will try and reply with any other random email account instead of the one it should. Occassionally some emails will use the right one but often not. Each email is different but once it has the different email address to use it won't change - if you save the email and click reply it will continue to use the one it has 'picked'.

I can only think it must be some sort of problem with Outlook Express' files or a screw up in the registry somehow.

Did you find a solution?