Yes. All outgoing messages sent from your account, including inbox responses, use credits. All incoming messages are free. For example,
You send a message to 500 people = 500 credits.
20 people reply back with questions = 0 credits.
You respond to each of the 20 questions = 20 credits.
Note: If the message is longer than 160 characters or includes emojis, a picture, or special characters, it may use more credits.