Currently, email postponed with the postpone command does not have the \Draft flag applied to it. Given that the purpose of the postpone command seems to be for saving drafts (the default folder for the command is "Drafts"), the flag should be added. I know currently aerc does not use this flag for the recall command, but webmail/other mail programs use this flag to display the message differently and allow re-editing. Mail currently postponed in aerc will function incorrectly in these other programs.