Port 174 sits in the well-known port range (0-1023), officially assigned by IANA to a protocol called MAILQ. But if you've never heard of MAILQ, you're not alone. This port is a fossil from the early days of Internet email—a protocol that was registered but never widely adopted, leaving behind only its name in the registry.
What MAILQ Was Supposed to Be
In the 1980s, email traveled through networks like UUCP (Unix-to-Unix Copy), a store-and-forward system where messages hopped from machine to machine using dial-up connections. Each computer maintained a mail queue—a directory of messages waiting to be sent to the next hop.
Port 174 was registered by Rayan Zachariassen, who pursued a Ph.D. in "Mailer Science" from 1987 to 1991 and contributed to email routing systems during that era.1 MAILQ was meant to be a protocol for remotely inspecting mail queues—checking what messages were waiting, where they were going, who sent them.
The problem: by the time anyone might have used it, the world had moved to SMTP and direct Internet connections. The store-and-forward model that made UUCP necessary was already fading. MAILQ became obsolete before it ever became relevant.
What Happened to MAILQ
The mailq command still exists on Unix systems today—it prints your local mail queue. But the network protocol on port 174 never caught on. No major mail server software implements it. No RFC defines it in detail. The port assignment exists, but the protocol behind it is essentially vapor.
What remains is an entry in IANA's registry: "mailq 174/tcp MAILQ" and "mailq 174/udp".2 A reservation for a protocol that never lived.
The Well-Known Port Range
Port 174 sits in the well-known port range (0-1023), reserved by IANA for system services and widely-used protocols. Getting a port in this range required official assignment. Someone thought MAILQ was important enough to deserve a permanent place in this namespace.
They were wrong, but the reservation stands. Well-known port assignments are essentially permanent. Port 174 will be reserved for MAILQ forever, even though MAILQ doesn't really exist.
Malware's Attempt to Squat
Security databases flag port 174 as having been used by trojans in the past.3 When a protocol dies, its port becomes an empty room. Malware writers sometimes try to move in.
This doesn't mean current traffic on port 174 is malicious—just that an abandoned port creates an opportunity. If nothing legitimate uses it, malicious traffic becomes harder to distinguish from the void.
Checking What's Listening
To see if anything is listening on port 174 on your system:
If something appears, investigate it. Legitimate software rarely uses port 174. If nothing appears, that's normal. This port is mostly empty.
Why Unassigned (and Dead) Ports Matter
The port number registry is a fossil record. Each assignment tells a story about what people thought the Internet would need. Some assignments—like 80 for HTTP or 22 for SSH—became permanent infrastructure. Others, like port 174, became memorials to ideas that didn't make it.
These ghost ports remind us that the Internet we have isn't inevitable. Every protocol, every port, every standard represents someone's attempt to solve a problem they saw in front of them. Most attempts fail. A few become the foundation everything else builds on.
Port 174 was someone's solution to inspecting mail queues in the UUCP era. The problem was real—system administrators genuinely needed to check mail queues remotely. But SMTP won, UUCP faded, and the need for a dedicated mail queue inspection protocol evaporated.
What remains is the port number, forever reserved, eternally waiting for traffic that will never come.
The UUCP Context
To understand why MAILQ existed at all, you need to understand UUCP.4 In the early 1980s, most computers weren't permanently connected to the Internet. They dialed into each other over phone lines, transferred files and email, and hung up.
Email addresses looked like duke!research!ucbvax!user—explicit routing instructions telling each machine where to forward the message next. This was called a "bang path" because of the exclamation marks.5
Each machine maintained a queue of outbound messages. When a connection succeeded, the uucico program would process the queue, sending everything waiting for that destination. When connections failed (often), messages sat in the queue until the next successful call.
MAILQ was supposed to let administrators remotely inspect these queues without logging into each machine. In a world of intermittent connections and store-and-forward routing, that was a reasonable thing to want.
But SMTP and permanent Internet connections made the whole model obsolete. MAILQ died with the world it was designed for.
The Truth About Port 174
Port 174 carries nothing. It's a reservation for a protocol that never mattered, assigned to a service that never shipped, waiting for traffic that never comes.
It's a gravestone with a name on it. The person it memorializes never really lived.
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