A letter is one person's written message to another pertaining to some matter of common concern while an emails are messages distributed by electronic means from one computer user to one or more recipients via a network. Letters and mails should contain your contact information.
Contact information is an individual's private or personal information by which another person, can use to reach the other individual person.
The thing that should be included in the closing portion of your letter or email is your contact information. You need to let the person you are talking to know how to contact you if they need to.
