Comment 1 for bug 1459490

this seems to be related to this comment, however it's not clear why this is true:

;; If you have postponed messages you will be asked if you want to continue
;; composing them, if you say "yes" you will visit the `vm-postponed-folder'
;; and you can select the message you would like to continue and press "m"
;; again! However be aware this works currently only if you expunge all
;; messages marked for deletion and save the postponed folder.