You cannot mandate how hard somebody must work. It doesn't work. Make it 'expensive enough' to be wrong, and *then* they will make the necessary effort to be 'right'.
Some people block mail from bad places in an attempt to hurt the bad place, i.e. in an etempt to make it expensive for them to be bad. But nowadays there are so many bad places, so much SPAM that leaks through filters, and so many missing emails, that it becomes harder and harder to hurt the bad places by blocking email. Nowadays it is normal for email to mysteriously bounce, to go missing, to get delivered days or months late. Soon Internet email will be like IRC, a quaint service for Internet enthusiasts and oldtimers, but not a useful tool for businesses or ordinary individuals. --Michael Dillon