After clicking Compose, you will be presented with the screen below where you can enter the following:-

To |
Here you enter the email address of the recipient, or you can click the |
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Subject |
Here you enter the subject (title) of your email message, it is advisable not to leave this blank as many email systems will treat it as junk mail (spam) if you do |
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Cc / Bcc |
The Cc box (under To+) can be used to send a copy to other recipients where all recipients can see who it has been sent too whereas Bcc (which you enable by clicking the Bcc button) hides additional recipients (which is what you should use if sending to multiple recipients) |
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Toolbar |
Use this toolbar to format your message i.e. change font style, font size, bold, underline text etc |
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Attach file |
Click this icon to attach a file (document/photo etc) to your email
You can also drag a file to attach it to the email message |
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Message |
Type the main body of your email message here |
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Send |
When you have finished composing your message click Send, or you can click Discard if you don't wish to continue composing the new email |