ETRN (Extended Turn) is an extension to the SMTP e-mail protocol. ETRN is defined in RFC 1985. It enables a mail server to request a second mail server to forward it outstanding mail messages. The second mail server should initiate a new SMTP connection back to the first server. Some level of security is provided by DNS-based authentication of the initiating server. See also ODMR. Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ... Wikipedia does not yet have an article with this exact name. ... A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ... The domain name system (DNS) stores and associates many types of information with domain names, but most importantly, it translates domain names (computer hostnames) to IP addresses. ... Authentication (Greek: αυθεντικός = real or genuine, from authentes = author ) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the thing are true. ... ODMR (On-Demand Mail Relay) is an extension to the SMTP e-mail protocol. ...