Unassigned by IANA, no known squatter — whatever answers here is site-local.
10621 falls inside the unassigned gap between Apple's serverdocs at 10548 and printopia at 10631, and Nmap's service list records it as unknown on both TCP and UDP. There is no product convention, malware family, or scanner signature publicly tied to this number. In practice ports in the 10000-11000 band get picked by hand for internal services because the famous neighbours — Webmin and NDMP on 10000, Zabbix on 10050/10051 — made the range familiar to admins. Treat the number as meaningless and read the banner.
Expose with care
Unknown service, unknown auth. Check what is actually listening (ss -ltnp on the host) before you open a firewall rule for it.
$ nmap -Pn -sV -p 10621 TARGETSeen 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.