The EDIT option is not available for emails in Drafts.

This appears to have been caused by an IMAP mailbox migration where a folder called Drafts and it's contents was copied to a new installation. The result seems to be that the IMAP folder created in the transfer prevents the normal function of the original Drafts folder.

As a workaround I moved the contents of Drafts to another folder (Old Drafts). I then deleted the Drafts folder and then saved a new email to Drafts. After signing out and back in again the new Drafts folder was available and the message could be edited.

Any messages moved from (Old Drafts) to the Drafts folder still cannot be edited.