Replying
When you get a message, you may wish to reply to it. That is, send a new message back to the author of the message. This is done by viewing the message and then clicking on the Reply link. You will then be presented with a screen that closely resembles the Compose Message screen, howevere there are some differences.
First of all, the message is already filled in with the old message, and it has been surrounded by some special codes that are used for quoting text. This is the [quote] code, and text within it will be indented. You can then write below it so that the recipient understands that you are replying to a certain part of the message you received.
The next difference is that the recipient field is locked; you cannot change it. This is to prevent people from "replying" to other people than they originally received the message from. If you wish to send the message to another person, use the Forward feature.
You will also notice that the subject is the same as the one that was used when you first received it, however the text "Re:" has been prepended to the subject. This is to let the recipient see that this is a reply to an earlier message.
Just like the Compose Message screen, you can tick the Show Signature checkbox to enable adding the signature to your message.