Taughannock Networks Apple Inc. When the 5321.MailFrom domain in an e-mail message has a DNS MX Resource Record (RR), it is making an explicit statement that it is willing to accept email. However, when the domain has just a DNS A or AAAA RR, there mail clients cannot easily tell whether the domain accepts mail, as many hosts on the Internet advertise an A or AAAA RR regardless of whether they want to accept email. The NULL MX RR formalizes the existing mechanism by which a domain announces that it accepts no mail.