cannot send message

I cannot send messages from my yahoo mail account. The messages stay in the outbox and i get the following error message. My gmail account works fine, and i can send from my yahoo account using their web interface and via other mail clients

Has this worked before?

If it has, can you check with Yahoo Support, because it seems that provider often has connection  or server issues that resolve after some time.

It has worked before. As I have explained, I can send mails from other clients (e.g. from my mac), and from the web interface. And eM works fine for my gmail. So, it seems to be a particular problem with the combination of yahoo mail and eM.