To quote Robert Miles in an answer yesterday:
"To see the incoming mail on both computers: Under Windows Mail,
click on Tools, then Accounts, then your email account, then Properties,
then Advanced. If there is no check mark in the box before Leave a
copy of messages on server, click on this box, then Apply. It's a good
idea to also select one of the methods of eventually deleting the messages
from the server, to keep it from getting so many that it refuses to accept
more, and then click Apply. Click OK, then Close. Do this on the other
computer also."