Click on the Compose toolbar icon opens the message compose screen. Depending on your settings it opens in the current browser tab or in a new window.
Enter at least one recipient address or select them from the address book, type in a subject and the message text and click the Send button in the toolbar when you’re ready to send the message.
Save it as Draft (toolbar icon) if the message is not yet complete and you’d like to finish and send it at a later time. The email will be stored in the Drafts folder and can be double-clicked there in order to resume composition.
While composing a message, copies are periodically saved to the drafts folder to avoid unintentional loss. In case the session terminates unexpectedly or your computer or browser crashes, you’ll find a copy of the message on the Drafts folder. You can configure the automatic saving interval in the User Preferences.