Port 57: MTP — The First Home of Email

What Port 57 Is Now

Port 57 is officially unassigned. IANA's Service Name and Transport Protocol Port Number Registry lists no current service for this port¹. It sits in the well-known port range (0 to 1023), the range reserved for services important enough to be assigned by IANA and typically requiring root or system-level privileges to bind.

If you find something listening on port 57, it is not running a standard service. It is either a custom application, a misconfiguration, or something you should investigate.

But port 57 was not always empty. It has a history, and that history matters.

The Protocol That Lived Here

In September 1980, Suzanne Sluizer and Jonathan Postel at the University of Southern California's Information Sciences Institute published RFC 772, the first specification of the Mail Transfer Protocol². Its objective: "to transfer mail reliably and efficiently."

Before MTP, email on the ARPANET was a feature bolted onto FTP. You transferred mail the same way you transferred files. It worked, in the way that duct-taping a mailbox to a forklift works. Sluizer and Postel wanted something purpose-built.

In May 1981, they published RFC 780, an updated MTP specification that removed all references to FTP and formally assigned the protocol to port 57³. MTP operated over TCP using a full-duplex connection. It transmitted 7-bit ASCII, one character at a time, high bit cleared to zero. It had commands: MAIL, MRSQ, MRCP, QUIT, NOOP, HELP. It used three-digit reply codes. If this sounds familiar, it should.

What Happened Next

Six months later, in November 1981, Postel published RFC 788⁴. The protocol was essentially the same, but he made two changes that would echo for decades:

1. He added the word "Simple" to the name. Mail Transfer Protocol became Simple Mail Transfer Protocol.
2. He moved it to port 25.

In August 1982, RFC 821 formalized SMTP as a standard⁵. Port 57's assignment was quietly revoked. The protocol that had lived here moved to port 25 and became the backbone of every email system on Earth.

SMTP still runs on port 25 today, over forty years later, carrying billions of messages daily. The three-digit reply codes from MTP survived the transition. The command structure survived. The model of sender and receiver establishing a channel and exchanging commands survived. Almost everything survived except the port number and the absence of the word "Simple."

The Well-Known Port Range

Port 57 falls within ports 0 through 1023, the well-known ports. This range is controlled by IANA and carries significance: on most operating systems, only privileged processes can bind to these ports. Being assigned a well-known port was, and remains, a mark of importance. It means the Internet's governing bodies decided your protocol mattered enough to reserve a number for it in a space with only 1,024 slots.

Port 57 had that status. It lost it. Not because MTP failed, but because it succeeded so well that it evolved past the need for this address.

How to Check What's Listening on Port 57

Since port 57 has no assigned service, anything listening here warrants attention.

On macOS or Linux:

# Check if anything is listening on port 57
sudo lsof -i :57

# Or using netstat
sudo netstat -tlnp | grep ':57 '

On Windows:

netstat -ano | findstr ":57 "

From another machine:

# Test if port 57 is open on a remote host
nc -zv <hostname> 57

If you find a service bound to port 57, identify it. It is not running a standard protocol. In security contexts, unexpected listeners on well-known ports with no assigned service are worth investigating.

Security Considerations

Port 57 has been flagged in some threat databases as a port historically used by certain trojan programs for communication⁶. This is not unique to port 57. Malware authors often favor unassigned well-known ports precisely because they are unassigned: no legitimate service competes for the port, and a firewall rule allowing "well-known ports" might let the traffic through.

Best practice: if port 57 is open on your systems and you did not intentionally open it, close it and investigate.

Why Unassigned Ports Matter

The Internet has 65,535 valid port numbers. Only a fraction are formally assigned. The unassigned ones are not waste. They are headroom, the space that allows new protocols to be tested, custom services to run, and the unexpected to find a home.

Some unassigned ports, like 57, are former residents. Their assignments were revoked when protocols were superseded. Others have never been claimed. Together, they represent the Internet's capacity for growth, the empty lots in a city that is still being built.

Port 57 is one of those empty lots with a foundation still visible. If you know where to look, you can see the outline of the building that once stood here: the first purpose-built home for electronic mail.