The default signature in all your devices is "Sent from my <iPhone, iPad, etc.>." You can customize your signature to say anything you want, and it will show up on all messages you send, reply to, and forward. Here's how to do it:
- Open Settings> Mail, Contacts, Calendar.
- Tap on Signature.
- Type in the signature you want to use.
- Tap on the Mail (or Mail, Contacts…) button in the top left corner of the screen to save your signature and go back to the previous screen.