ETRN
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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. The ETRN operation is useful for intermittently connected mail servers. See also ODMR.