Here's a couple of tips I picked up:

Highlight the hotmail icon in the left hand window first then select send
all from the send and receive menu.

or: ctrl+click on the hotmail icon in the left hand window and select empty
cache from the drop down menu then do the same and select get complete
folder list - it should send now but still use the method above

You can also try going to the tools menu and then accounts and edit your
details so that the account name is the same as your email address - no idea
why this works but it does seem to.