Postfix ETRN Howto
Purpose of the Postfix fast ETRN service
The SMTP ETRN command was designed for sites that have intermittent
Internet connectivity. With ETRN, a site can tell the mail server
of its provider to "Please deliver all my mail now". The SMTP server
searches the queue for mail to the customer, and delivers that mail
by connecting to the customer's SMTP server. The mail is
not delivered via the connection that was used for sending ETRN.
Postfix versions before 1.0 (also known as version 20010228)
implemented the ETRN command in an inefficient manner: they simply
attempted to deliver all queued mail. This is slow on mail servers
that queue mail for many customers.
As of version 1.0, Postfix has a fast ETRN implementation that
does not require Postfix to examine every queue file. Instead,
Postfix maintains a record of what queue files contain mail for
destinations that are configured for ETRN service. ETRN service
is no longer available for domains that aren't configured for the
service.
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