Are PTR Records Important?

erfmuffin asks: "I work for a medium-sized regional ISP. Recently we configured our email gateway to refuse connections to IP addresses that do not resolve (ie no reverse DNS). I am amazed at how many legitimate domains use mail servers with no PTR record! At the same time, we have avoided a great deal of junk mail in one swoop. Wouldn't it be better for mankind if all mail servers refused mail from non-resolvable IPs? Should all legitimate mail servers have valid PTR records or has the world become too lazy to make email delivery, easier?"