On-Demand Mail Relay — an intermittently connected mail server pulling its queued mail from an ISP.
ODMR, specified in RFC 2645, exists for domains that are not always online. The client authenticates to its provider with SMTP AUTH, issues the ATRN command, and the two ends then swap roles so the provider delivers the mail it has been queueing over the same connection. It was built for dial-up and intermittent links, which is why it is nearly extinct — always-on connections and POP/IMAP collection replaced it. A live 366 today is usually a small ISP or hosting provider that never turned the service off.
Expose with care
ODMR requires SMTP AUTH before ATRN, so it is only as safe as the TLS around it; without STARTTLS enforced it hands over mail credentials in the clear.
$ printf 'QUIT\r\n' | nc -v -w 5 TARGET 366Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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