Why can Sprint/Nextel users reply to my ShoutOUT messages?
This is a consequence of the way the Sprint/Nextel network handles SMTP messages. ShoutOUT text messages-whether voice- or keyboard-generated-are sent in SMTP (email) format, not standard SMS format. This is why they can be sent at little to no cost. Sprint/Nextel does not permit its customers to reply to messages received through its SMTP / SMS gateway. You can send ShoutOUT messages to Sprint/Nextel subscribers, but in order to reply, they will have to send a regular SMS message to your mobile phone number.