Hi, first of all a great plugin
SORRY THIS SHOULD BE PART OF http://wordpress.org/support/topic/failed-to-send-message-1
I had a funny problem
Contact form would not send a message if user was logged out
error message was in a red framed box "Failed to send your message. Please try later or contact the administrator by another method."
I then logged in, could send a message successfully. After this action I could then send messages from logged out state and everything functioning OK again