Windows Live Mail (Windows 7.0): messages have started sticking in Outbox but only on 1 account

We have 2 Live Mail accounts on one pc. Suddenly one of them is having trouble sending messages (all types) . They get stuck in the Outbox. To get them sent I have to log off and log back in, and when I do the messages are sent from the Outbox as I open Live Mail. This means I can only send messages when I have freshly signed on.......after that they stick. I have tried using the repair function on Windows essentials and also resetting Internet Explorer (someone recommended this). Makes no difference if the messages have attachments or not. It happens to all types.

Any ideas?