Apache SpamAssassin's scanning daemon, which trusts anything that can reach it.
spamd is the long-running SpamAssassin server that mail systems hand messages to for scoring, avoiding a Perl startup per message; its documented default listening port is 783 and clients such as spamc connect to it. It is not IANA-registered — 783 is SpamAssassin's own convention. The protocol has no authentication: any client that can connect may submit mail for scanning, and where --allow-tell is enabled it can also drive Bayes learn/forget commands. Default configurations bind localhost, so an externally open 783 is a misconfiguration.
Do not expose
Unauthenticated, and it costs real CPU per submitted message; bind it to loopback or a private interface behind the MTA.
$ nc -vz 127.0.0.1 783Seen 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.