You can save your message as a draft to be completed and sent at a later time. Draft messages are saved in your Draft folder until you send them or delete them from the folder.
You can enable a preference to automatically save your email as a draft while you are composing the message. You can also click Save Draft while composing to save the message as a draft
Go to the Preferences>Composing tab and enable Automatically save drafts of messages while you are composing. Messages you compose are automatically saved as a draft every few minutes.
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, or select the message and click Edit in the toolbar.
You can compose and send the completed message as normal. When sent, the message is removed from the Drafts folder.