1. Go to the previously received e-mail and click "reply". You can then change the e-mail address to the person you want to send to (if not replying). You can also change the subject line to remove the "Re" or whatever which signifies a reply. Attachments may not be included if the previous e-mail has them.
2. If the previously received e-mail has an attachment, you can click "forward". You can then enter the appropriate e-mail address and modify the subject line as desired.
3. If you've already composed the e-mail you wish to send and don't wish to retype it, just copy/paste the composition into the reply or forward.
Yes that works for one email with the exception of the first emails attachments. Sometimes those attachments are essential to the communication. The other short fall is if one needs to attach more than one previous email to the email being composed.