Unassigned at IANA, but widely squatted as an alternate SMTP port when ISPs block 25.
IANA lists port 26 as Unassigned with no registration on TCP or UDP. In practice it is one of the most common squats among shared-hosting providers, which run a second SMTP listener here so customers on residential connections whose ISP blocks outbound 25 can still send mail. It also turns up as an alternate SSH port on hosts that moved sshd off 22 to cut brute-force noise. Neither use is standardised, so what answers on 26 has to be fingerprinted rather than assumed.
Expose with care
When it is an alternate SMTP listener it usually carries the same cleartext-auth risk as port 25; require STARTTLS or prefer submission on 587.
$ nc -vz TARGET 26Seen 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.