Summary

  • RFC 821's optional TURN command reversed the client and server roles on an existing SMTP channel, helping intermittently connected hosts collect queued mail but allowing an asserted host name to control where that mail went.
  • RFC 1985's ETRN made the request smaller: the caller could ask a server to start a named queue, while the server kept local authorization, opened a separate outbound connection and returned before delivery was known.

The cheapest connection had to serve twice

SMTP was designed when a permanent inbound path could not be assumed. A small site might dial a service provider, send its outgoing mail and want the provider to return mail that had accumulated while the site was offline. Waiting for the provider's ordinary retry timer wasted the rare interval in which the site was reachable.

RFC 821 offered an economical answer. After the caller had acted as sender-SMTP and the provider as receiver-SMTP, the caller could issue TURN. If the provider replied 250, both programs changed roles on the same transmission channel. The caller became the receiver, sent a fresh service-ready greeting and waited for the provider to deliver its queue. The provider could refuse with 502, and the command was optional.

The mechanism reused a scarce link. It also reused more trust than the link had earned.

A name spoken in HELO was not custody proof

The server needed to decide which queued mail the new receiver should obtain. Yet early SMTP did not authenticate the host name supplied by the peer. A rogue caller could present another site's name, request TURN and invite the server to send that site's waiting mail down the open channel.

This was not merely a forged line in a header. It could change custody of entire stored messages. RFC 1985 later called the defect a large security loophole and observed that many SMTP and ESMTP implementations avoided TURN because the specification supplied no check on the asserted remote name.

The problem was the size of the transition. One unauthenticated assertion changed the caller's role, the server's role, the direction of the existing connection and the destination of queued mail in a single 250 decision.

ETRN requested work without receiving it

RFC 1985 kept the operational need and narrowed the grant. A server that supports the extension advertises ETRN after EHLO. The client supplies a node name and asks the server to start processing the corresponding queue. It can make the request after a session is established, but not inside a mail transaction between MAIL FROM and the completion of DATA.

The server inspects the requested scope and makes a local administrative decision. If it accepts, it starts its retry queues and uses another SMTP connection to send toward the named remote site. The original caller remains the caller. It does not become the receiver on the control channel, and no stored envelope is handed to it simply because it knew a name.

The separate connection is not magical authentication. It is a new evidence boundary. DNS, route selection and the receiving SMTP transaction decide where the server can connect and whether each message is accepted. Those checks are imperfect, but they are independent of the trigger connection's bare claim.

A successful trigger was not a receipt

Queue work can take an indeterminate time. RFC 1985 imposes no requirement that a new connection occur within a given period. The command therefore returns promptly rather than holding the control session open until the queue drains.

The ordinary 250 response means the request was satisfactory and queue processing started. It does not prove that messages were waiting, that an outbound connection succeeded or that any recipient accepted a message. Optional 251, 252 and 253 replies can reveal progressively more queue information, but they remain reports from the queue-holding server, not end-to-end delivery evidence.

This separation matters operationally. A monitoring system that records an ETRN 250 as “mail delivered” destroys the very uncertainty the extension preserved. Trigger acceptance, attempted connection, message transfer and final delivery belong to different records.

Queue names stayed under local authority

The simplest ETRN parameter is a fully qualified node name. RFC 1985 also defined @domain to activate queues for the named domain and its subdomains, and #name to select a locally defined queue such as a UUCP queue.

These conveniences expand scope. A careless suffix such as @com could release enormous work and create congestion. A local # name has no global dictionary, and the protocol offers no method to enumerate all queues. The server must authorize caller-and-scope combinations that make sense for its own storage model.

Syntax therefore coordinates a request without nationalizing the queue. IANA can register ETRN; it cannot decide which customer may wake which workload.

Deprecation recorded a reduced trust budget

RFC 2821 deprecated TURN, and RFC 5321 retained the verdict: SMTP systems should not use it unless the server can strongly authenticate the client asking for the role switch. Encryption alone would not establish that entitlement; the relevant question is whether the authenticated principal may receive the particular queued mail.

RFC 5321 also described a still smaller optimization. If mail arrives from a host, a server may treat that contact as a hint to retry mail queued for the same host. No explicit “send queues now” command is always necessary. The hint can influence local scheduling without becoming remote ownership of the queue.

The current IANA registry preserves both historical names while marking TURN and ETRN as unsuitable for the message-submission service. A user submitting mail on port 587 is not thereby entitled to activate an SMTP delivery queue.

Internet mail did not abandon intermittent hosts. It stopped making their convenience depend on one oversized role transfer. The caller could trigger work. The queue holder could authorize and schedule it. A separate SMTP path could test delivery. None of those facts had to impersonate the others.

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