"Send Message" transmits your e-mail message to the recipient(s). Make sure you want to send your message as this action is final and cannot be reversed.
"Save Draft" places an unfinished message in a drafts mailbox, accessible through any folder list. You can then finish the message later by switching to the drafts mailbox, clicking on the saved message to open it, and then clicking on "Resume".
Entering a Subject in the e-mail before saving as a draft will allow you to more easily identify later which message you want to resume within the drafts mailbox."Cancel Message" drops the compose message in progress and, if composing in a non-popup window, returns you to the last mailbox you were viewing.