feedmail-queue-runner-cleaner-upper

feedmail-queue-runner-cleaner-upper is a variable defined in `feedmail.el'.
Its value is

(lambda
  (fqm-file &optional arg)
  (delete-file fqm-file)
  (if arg
      (feedmail-say-chatter "Nuked %s" fqm-file)))

Documentation:
Function that will be called after a message has been sent.
Not called in the case of errors. This function is called with two
arguments: the name of the message queue file for the message just sent,
and the optional argument used in the call to `feedmail-run-the-queue'
or `feedmail-run-the-queue-no-prompts' (prefix arg if interactive).
In any case, the affiliated buffer is killed elsewhere, so don't do that
inside this function. Return value is ignored.

The default action is an anonymous function which gets rid of the file
from the queue directory. With a non-nil second argument, a brief
message is give for each file deleted. You could replace this
function, for example, to archive all of your sent messages someplace
(though there are better ways to get that particular result).

You can customize this variable.