This is the computer that runs the mail program and actually sends and receives your mail, then puts it into your mailbox.
All mail programs have a cc option somewhere.
It's not considered polite to use this except when you want to send a blind cc to yourself. It's especially useful if you have two mailboxes and don't want the recipient of your message to get confused. Not all mail programs have the bcc option. Some mailing lists won't let you post a message without a subject, and some mail programs are intelligent enough to remind you if you try to send a message with no subject. Message What you're trying to say.