The following options exist:
saving an E-Mail as a draft while you are composing the E-Mail
sending a saved E-Mail draft
How to save an E-Mail as a draft:
Click on the Save button on the Compose page.
Result: The E-Mail is saved in the Drafts folder.
How to send a saved E-Mail draft:
Open the Drafts folder.
Select an E-Mail.
Click on Edit draft in the toolbar.
Complete the entries. Click on Send.