How are email headers generated?

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http://unix.stackexchange.com – When you send an email a header is part of the message. For example, when you check your mail you can view the raw source and see the headers, which has information about the sender of email in it, etc. When I send mail using the mail command, from address1 to address2, the header includes some information about the sender, like sender's hostname, IP, etc. Now my Question is: How is the information for the header generated, and how can I avoid or change them (i.e, forge them)? I'm use Fedora 17 and Sendmail. (HowTos)