IMP says "there is an error sending your message"

[York College]

Hi,

I am new to IMP. I installed php4.44, horde2.0, imp3.0. After
I loged in from imp, I can see the email I received. However,
I can't send out email from imp (I can send out email using
other mail tools). When I tried to send, I got "there is an
error sending your message" on the top of my compose window.
My email server, imap, imp are on the same machine.

Any clue will be very much appreciated.

Laura