At multiple gigs, at multiple sites, a significant amount of bounced mail consists of messages sent to long-term undeliverable addresses (in many cases: to domains which no longer exist, and/or have been tranferred, and/or the owning company has gone out of business: think Enron, AT&T's discontinued ISP network, Lehmann Brothers, etc.).
Even if I'd _wanted_ to create rules or write scripts to automatically process the messages, the login requirements generally meant that wasn't possible. Instead, these comprised both a significant amount of outbound mail queues and nondelivery notifications, potentially masking more serious issues (you've got to come to understand what notifications are effectively part of background noise vs. not).
Oh, and some of those domains still exist in some regards (e.g., there's a skeleton crew at Lehman winding down the firm), so you can't just blindly select entire domains.
File under continuing hassles of a conscientious admin's job.