Can receive email but can't send any.

Hi,

I'm unable to send emails although I can receive them. When I press send the email seems to just dissapear there is no email in the sent messages folder, no delivery failure message, no email received on the other end. Strangely though when I'm in the sent folder and I press the send/receive button the messages that I have attempted to send briefly cycle through the box below.

Has anyone else encountered this or have an idea what the problem may be?

Many thanks

Lizzie