feedmail-queue-use-send-time-for-date

feedmail-queue-use-send-time-for-date is a variable defined in `feedmail.el'.
Its value is

nil

Documentation:
If non-nil, use send time for the Date: header value.
This variable is used by the default date generating function,
feedmail-default-date-generator. If nil, the default, the
last-modified timestamp of the queue file is used to create the
message Date: header; if there is no queue file, the current time is
used. If you are using VM, it might be supplying this header for
you. To suppress VM's version

(setq vm-mail-header-insert-date nil)

You can customize this variable.