You can save your message as a draft to be completed and sent at a later time. Messages are saved in your Draft folder until you send them or delete them from the folder.
Compose your message as described in Creating a new mail message.
Click Save Draft.
The draft message is saved to your Drafts folder.
Open the Drafts folder, click on the message.
You can compose and send the completed message as normal. When sent, the message is removed from the Drafts folder.