Here's a question that has always been bugging me.
Ever since we moved from smail to exim many years ago at my isp, exim
never seems to discard messages in the input queue.
Even though the single retry rule is the stock one (which retrys for
something like 4 days), we end up with stuff that is weeks... months
old. Periodically, it would get pretty full and we'd notice that there
were about 10 queue runners going and so I'd go in and do a "find" and
remove anything older than 14 days or so. I *had* to do a find, because
doing an "ls" would just sit there an churn for about a half-hour.