A DNS Domain Name Server: A server that answers queries regarding the names and addresses on the internet. server is used to convert a text server name into its numeric IP address and to return other information required for mail handling. At least one DNS server is required if you intend to use any of the following features:
RBL Real time Blackhole Lists: a list of mail server that are considered sources of SPAM by the list owner lists (SMTP Simple Mail Transfer Protocol (A message send protocol) and Filter policy)
SMTP PTR A term used for the reverse pointer record in a DNS system. This allows a server to determine the name of a computer from its IP address. record checks
SMTP SPF Sender Policy Framework: A system utilising DNS servers to validate that a given IP address is authorised to send mail for a specific domain checks
MX delivery of outbound mail