There are over 2500 RFC's. Each RFC is known by a number. For instance, RFC 1180 presents a tutorial on TCP/IP, RFC 1920 lists the current standards RFC's and explains the RFC standards process, and RFC 1941 is a FAQ list on the topic of Internet deployment in educational establishments.
MX record. A mail exchanger record (MX record) is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available.
Techopedia explains Microsoft Exchange Server (MXS) In the 1990s, email evolved into a business-critical application, leading to the development of user-friendly enterprise solutions with improved features and connectivity. The most current version, Microsoft Exchange Server 2013, allows a user to deliver email, contacts and calendar to a PC, mobile device or browser.