Outlook Express imported mail from Yahoo.

I use Yahoo as my normal email application. I use Outlook Express occasionally so that I can put a photograph in the message body.

When I opened Outlook Express this evening it imported all the messages from my Yahoo inbox - without me telling it to. This is most annoying.

Is there 1) any way to get each item back into my Yahoo inbox? and 2) is there any way to stop Outlook Express from doing this again?

And why did the damned thing do it anyway?