feedmail-queue-auto-file-nuke

feedmail-queue-auto-file-nuke is a variable defined in `feedmail.el'.
Its value is

nil

Documentation:
If non-nil, automatically delete queue files when a message is sent.
Normally, feedmail will notice such files when you send a message in
immediate mode (i.e., not when you're running the queue) and will ask if
you want to delete them. Since the answer is usually yes, setting this
variable to non-nil will tell feedmail to skip the prompt and just delete
the file without bothering you.

You can customize this variable.