Windows RPC tunnelled over HTTP — the endpoint mapper for RPC/HTTP, historically Outlook Anywhere.
IANA registers TCP 593 as "http-rpc-epmap": the HTTP RPC endpoint mapper. Microsoft's RPC over HTTP protocol ([MS-RPCH]) tunnels DCE/RPC traffic through an RPC-over-HTTP proxy so it can traverse firewalls, and 593 is the endpoint-mapper counterpart to the plain DCE endpoint mapper on TCP 135. Its best-known consumer was Outlook Anywhere against Exchange; the proxy itself normally publishes over 443, so 593 turning up on a scan is a Windows server exposing the RPC/HTTP plumbing directly.
Do not expose
This is an RPC control plane. Every reason TCP 135 is blocked at the perimeter applies here — clients should reach RPC/HTTP through the HTTPS proxy on 443, never this port.
$ nc -vz -w 3 TARGET 593Seen 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.