The message sending options are hidden by default and first need to be expanded by clicking the down-arrow on the left border of the message headers block.

There you can switch the editor type to compose html messages and adjust other settings concerning the message delivery.

The priority flag of the message to be send. This will be displayed in the recipient’s mailbox.
Return receipt
Request the recipient to return a receipt confirmation when opening the message. This will be sent by the recipient’s email program if she confirms it.
Delivery status notification
Request a notification message when the message is delivered. This will be send by the email server upon message delivery. Please note that this doesn’t confirm that the actual recipient has received the message.
Save sent message in
Select the folder where to save a copy of the message after sending it. This defaults to the Sent folder and can be changed here. Select don’t save to just send the message without saving a copy.