My servers have lots of resource and bandwidth that I can offer pop before smtp , however, I always tell customers to use their isp's outgoing mail server so that they won't overwhelm my machine. Unfortunately, few clients resell my hosting package to someone else and they don't wan't to explain their customers how to setup isp's smtp, they claim "customers change isp so often and I don't want to explain them when they can't send mail". pop before smtp will also cause me work harder when someone send spam, virus etc.