This morning, I found 1.6 Gigabytes of mail in my inbox. Before concluding that I'd need at least 2 GMail accounts, I looked more closely to find that the last 2000 or so messages were from my old friend Mailer-Daemon. The POP server to which I forward my mail for remote pickup had instituted a new anti-virus program that was bouncing some spam back to me, starting an endless loop of bounces and re-bounces that didn't stop until I changed my procmail rules to quarantine them.
Now I'll be the first to admit I have an idiosyncratic mail setup. I like to see (or be able to see) every single piece of mail sent to me, but I like to be able to read the good stuff on remote machines and devices like the Sidekick. I collect it all at my server, filter and munge it with procmail and spamassassin, and bounce it back out into a set of POP mailboxes. Which works great until my POP provider starts "adding value" to its services by filtering for viruses, and catches one that my home-grown filters haven't yet caught up with. It sends the message back to me with the message "you may have a virus." My filters see that message addressed to me and send it out for delivery to the POP box -- where those filters again bounce it back -- ad infinitum.
Of course a simple addition to my procmail rules fixed the problem, and a more cleverly designed set might have averted it altogether, but I'm still reminded of the value of truly basic service. "Enhanced" services will never account for all the idiosyncracies of their users. For some subset of users, the enhancements will require work-arounds that in turn require their own work-arounds, adding layers of complexity more than if the users could choose their own local enhancements to a basic service.
Network providers rarely know exactly how their users work. When they try to guess, they break things. The best network is one that does what it offers, and only that.