When viewing an individual email, you have the option to take actions on it using the Actions menu in the top-right corner of the email. One of these actions is Send a copy.
Use Send a copy when you want to send this message to a new address, exactly as it appeared to you. This is different from forwarding: When you forward an email, it shows that it was sent by your account, and the original message is included within. When you send a copy, the recipient sees the exact same message you see.
Note that the delivery headers are different in the copy compared to the original message, because the copy was sent by you. It can be compared to photocopying a paper letter, sticking it inside a new envelope, and sending it to someone else. The letter is an exact copy of the original, but it has new delivery instructions to send to a new person. Therefore, your username will be included in the headers of any email sent this way.
Why don't I see a copy in my Sent folder?
When you send a copy, it is exactly the same as the original, and you already have the original. The only difference is in the mail delivery headers, which are not normally displayed by most email programs, including our web interface.
If you need to know your mail has been sent, and to be able to easily see who you have sent it to, forwarding mail as an attachment is probably what you need.