If you receive email using the SMTP protocol and your server is not permanently connected to the Internet, email can be accumulated at another SMTP server (typically a secondary server for a given domain). When it is connected to the Internet, the SMTP server sends an ETRN command (command of SMTP protocol) and asks for stored emails to be transmitted. If the given SMTP server doesn't have any messages stored, it doesn't need to reply at all. That's why it is necessary to define a timeout period. If the SMTP server doesn't receive any emails, it terminates the connection after the specified timeout.

The ETRN command is used when an SMTP server is not online 24 hours a day. An ETRN enabled SMTP server will hold messages in the queue until another SMTP server uses the command to have them flushed.

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.