mutt deletes all in sent file

Greetings. I'm using mutt 2.4.1 with mbox on artix linux.  When I load up mutt,
press <Esc>c? and select the sent file, every sent email is displayed
with the D)elete mark before it.  If I select any file other than the
sent file, that does not happen.  Unless I U)ndelete every email in the
sent file, exiting will delete those emails.  What setting can I change
so that those emails are not automatically marked for deletion?

	ax