iPhone – Save E-mail Drafts
If you want to save a message so you can come back to it later, tap on Cancel, Mail will pop up a dialog box with the options Save, Don’t Save, or Cancel.
Tap on Save and the message will be placed in your Drafts folder. If your account doesn’t currently have a Drafts folder, Mail will create one. It does however sometime take several minutes for the draft message to show up