mysms provides you with two sending options (longpress on the send button to change the channel):

• Texts via mobile carrier (grey send button)
  In case you want to text people who don’t use mysms, you can choose to send via your mobile carrier (grey send button). Your existing mobile agreement will be used, so there are no additional costs for using this channel.
  
  
• mysms friends messages (green send button)
  With mysms friends you can send free messages to other mysms users. These messages are delivered worldwide and are sent via data connection. You’ll receive a free confirmation (check mark) that the message has been received. mysms recognizes whether the recipient uses mysms as well. In this case the send button turns green.