Reverse DNS Is Not A Valid Hostname

If MXToolbox reports that the reverse DNS is not a valid hostname, then you need to make the following change to your reverse DNS record: Contact your ISP and get them to change the PTR (reverse DNS) record to the A record set up in your DNS. For example, the reverse DNS should not simply be pointed to the domain sending your email, rather it should point to a valid hostname within that domain, such as smtp.domain.com, rather than domain.com.