Registered to IANA as "any private mail system" — a deliberately open-ended slot, not a specific protocol.
IANA's entry for port 24 is literally 'any private mail system', assigned to Rick Adams, meaning the number was set aside for private mail transports rather than any one named protocol. There is no RFC and no reference implementation. In practice almost nothing binds it, and when it appears on a scan it is usually a bespoke or legacy mail relay rather than a standard MTA. Standard mail lives on 25, 465, 587 and 993 instead.
Expose with care
No defined protocol means no defined security properties; if a private mail transport is really listening here, assume cleartext until proven otherwise.
$ nc -vz TARGET 24Seen 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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Corrections or a missing port? Reply @rimrocksystems.