Trouble sending emails with Horde

Some, but not all, of the emails I want to send with Horde simply won't go through. I compose the message than hit 'send' and the message box just recycles with my message appearing in text format. I hit send again and the same thing. I close the message box and check 'sent mail' and there is no record of the message being sent. This even occurs with recipients I have already been exchanging messages with. Any ideas?

Thanks,

Joe