Though this is an extremely basic usage note, we couldn't find documentation of it elsewhere and the process for execution isn't very intuitive. We recently received an email from an iPhone Atlas reader asking when the ability to save draft email messages would be added to the iPhone. This functionality is already there, but it's concealed.
To save a draft message, open Mail then tap the Compose button in the lower-right corner. Compose your message, then click the "Cancel" button instead of "Send." This will bring up a dialog box with the options "Save" and "Don't Save." Click the "Save" button, and your message will be saved to the Drafts folder for the designated account.