By default, when you send an email from your iPhone, a Sent from my iPhone signature will be added to your message.

You can edit this following these easy steps:

From your iPhone home screen go to the Settings menu.
Tap on the Mail icon.
Select Signature.
There, you can modify the signature for all mail accounts you have set up, or on a per-account-basis.
If you wish to have no signature, simply delete the entire text.

When ready, tap Mail on the upper left corner to save your changes.