This feature was introduced in 1.7f, not 1.8a. The change in behaviour is
due to LMail 1.2a which translates FAL delivery errors to a format
LISTSERV can parse and allows most delivery errors to be processed
automatically. You can turn this feature off with "Auto-Delete= No". Note
that only hard errors cause deletion. Some systems incorrectly report
temporary errors as hard errors but there's nothing LISTSERV can do about
it.
Eric