Businesses that ask for email addresses know that a significant percentage
of people can't type their own email address correctly. Each of those
results in a bounce, or an undeliverable message sitting in an mqueue
somewhere. It would not surprise me if they also reduced their
Timeout.queuereturn to a few hours as well.
Dealing with the bounces would be a nightmare, they've already got their
handsful with the webservers and the outbound mail boxes.