When you click Save to Drafts, rather than Send while composing a message, the message draft is saved in the Drafts folder and can be re-opened for further composing, editing, and sending at a later time if necessary.
Auto-save to drafts will automatically activate depending on your User Options setting for Composing Messages > Auto-Save To Draft Frequency (set by default to 10 minutes).
Caution: It is recommended to Save to Drafts to prevent any loss of data, should any system/software malfunction occur.
To edit a draft email: