Selecting the Auto Response link in the top left of the screen allows you to create a message that will be sent to each mobile number that sends a message to your account.
Each message will be counted as one or more messages sent (as if it had been written manually from the Compose screen) and a copy of the reply will be saved in the Sent Messages account with the username of System User.
Premium accounts will have the ability to set up different messages for the first inbound message from each number, a subsequent response each time that number sends in again and a stop message when the customer unsubscribes.