The default mail's signature on iPhone is "Sent from my iPhone". But changing the default signature on iPhone is as easy as touching the screen a few times. To change and add a signature on iPhone Mail application, do as follows:
1. Open the Settings application from the home screen
2. Go to the list of default iPhone's applications and tap to open the Mail, Contacts, Calendars settings
3. Scroll down to Mail settings and tap the Signature option
4. Tap the current signature and delete or change the signature on iPhone mail app.