5s does not send any messages, receiving without problem

Hi, my iPhone 5c (iOS 8.3) refuses to send any message, it does however receive incoming messages without problem. This occured suddenly from one day to the next. I have iMessage activated and when I send a message from the iPhone to my own e-mail account, it will actually show up in the messages. Strangely enough I can then reply from my iPhone to my own iPhone (but not forward to another number). I have tried turning the iPhone off and then restart, but that didn't help. I am not aware that I changed something in the settings inadvertedly. Does anyone have an idea? Thanks for any help!