Webmail Issue

[Shorts]

I seem to be having an issue with webmail on plesk 7.5.3. When trying to send a message via webmail i get the following message:

Error There was an error sending your message: unable to add recipient [kyodai_shorts@hotmail.com]: Invalid response code received from server

Works fine when i restart it, but i dont want to have to restart it every day.

Any Ideas