Send a copy
When viewing an individual email, use the More button to get to extra actions you can take. This includes the "Send a copy" feature.
Use "Send a copy" when you want to send this message to a new person, exactly as it appeared to you. This is different to forwarding: when you forward a piece of mail, the recipient sees the mail was sent by you, with the other message included inside. When you send a copy, the recipient sees the exact same mail you see.
Note that the delivery headers are different in the copy compared with the original mail, because the copy was actually sent by you. It's a bit like photocopying a paper letter, sticking it inside a new envelope and posting it to someone else. The letter is an exact copy of the original, but it has new delivery instructions to send to a new recipient.
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, mail forwarding is probably what you need.