You can replace the default “Sent from my iPhone” signature with a custom sign-off in your iPhone or iPad settings.
Open Settings.
Tap Mail. On older iOS versions, this may appear under Mail, Contacts, Calendars.
Tap Signature.
Edit the signature, then return to save the change.
New emails will use your custom signature instead of the default. If you have multiple mail accounts, you can set a signature for each account on the Signature screen.