We support all of those mail-sending methods and because of that we have not relayed one single message for a non-customer in the life of our service.
Maybe in part because your customers are remarkably endowed with clueons. In my abundant experience supporting configuration changes on the part of dial-up users (like Mindspring's), I have found that your average dial-up user is about as capable of reconfiguring and upgrading software as you or I am of building a nuclear reactor with a pencil, some sand, and a stick of bubble gum. Imposing security measures or performance enhancement tricks after initial implementation is a huge imposition on any company's technical support staff, and frequently serves more as a customer irritant than anything else. I remember having to assist flash.net customers with reconfigurating their POP3 and SMTP clients when that provider went to a round-robin load-balancing mail server system. It was ... painful. ag