I am trying to send a text to my friend who had an iPhone and we have been iMessaging for multiple days in a row and today it just started not delivering but can send as texts and he will receive them and his messages come in as iMessages.
The Messages app on your iPhone will revert to using SMS texting when it can't send messages via Apple's iMessage system. This could be for a number of reasons. One being loss of Internet connectivity (regardless if that connectivity is via Wi-Fi or cellular data.)
iMessage help
