can receive but can't send in windows mail

Hi hope someone can help with this.

I'm using windows mail in windows vista home basic. Since yesterday, I can
receive emails as normal but I haven't been able to send. When I click Send
the message goes to the Outbox and stays there. If I double-click the message
in Outbox, it opens in a new window, when I click Send again it remains
unsent and the message "Message could not be displayed
Windows Mail encountered an unexpected problem while displaying this
message. Check your computer for low memory or low disk space and try again".
As far as I can tell I'm not short of memory or disk space.
I've looked in Tools and the Outgoing server authentication box is checked.

Any solutions please?