Be default once you have set up your iPhone e-mail, each message that you send from your iPhone has ‘Sent from my iPhone’ appended to each e-mail you send.
This can be kind of pretentious, particularly for your friends/work colleagues who don’t have an iPhone.
Try personalizing this Signature by:
Tapping Settings
Mail, Contacts, Calendar
Signature
Delete the default message and enter something a little more personable.
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But can you set a signature for text messages on the iPhone??
Check out http://iphonenewbie.com/how-to-add-signature-to-iphone-txt-messages to use the keyboard shortcuts in iOS5 to do this for non-jailbroken phones.