Port 45 is assigned to the Message Processing Module, or MPM. You have almost certainly never encountered it. No modern software listens on port 45. No firewall rule specifically addresses it. But it is not unassigned. It is something rarer: a port that was given a purpose, served that purpose, and then watched the world move on.
What the Message Processing Module Did
The MPM was a message routing system, defined in RFC 7591 by Jonathan Postel in August 1980. It was designed to solve a problem that seems obvious now but was genuinely hard then: how do you send a message from a person on one network to a person on a different network when those networks have completely incompatible message formats?
The answer was a relay architecture. MPMs were processes that sat on different networks and forwarded messages between them. A user would compose a message through a User Interface Program (UIP), hand it to the local MPM, and the MPM would figure out how to get it to the destination. If the MPM could reach the recipient's network directly, it would deliver. If not, it would relay the message to another MPM that was "closer" to the destination, or fall back to what the RFC calls a "big brother" MPM with broader routing knowledge1.
Port 45 was the default TCP port where MPMs listened for incoming connections from other MPMs. Port 46 was designated for MPM sending (mpm-snd)2. The receiving MPM accepted the connection, parsed the message structure, and either delivered it locally or forwarded it along.
How It Worked
Messages in the MPM system had three parts: an identification section (transaction ID and the originating MPM), a command section (delivery instructions and a routing trace showing every MPM the message had passed through), and an optional document section containing the actual content1.
The protocol supported more than plain text. The specification anticipated "facsimile data, digitized speech, [and] graphics" as message content1. In 1980, Postel was designing a multimedia messaging system.
The MPM used TCP as its reliable transport layer. A pair of MPMs would establish a full-duplex connection, exchange messages and commands, and close the connection. Messages could be held at intermediate MPMs if the full path to the destination was not available simultaneously, making it a store-and-forward system1.
The History
RFC 759 was the second edition of the Internet Message Protocol specification. The first version appeared as RFC 753 in March 19793. Both were authored by Jon Postel at the Information Sciences Institute at the University of Southern California, as part of the ARPA-sponsored Internetwork Concepts Research Project.
The problem was real. In the late 1970s, ARPANET was not the only network. Organizations had their own internal messaging systems with incompatible formats. The MPM was designed to bridge these systems, converting messages at the boundaries between networks.
Then, in August 1982, Postel published RFC 8214: the Simple Mail Transfer Protocol. SMTP. Port 25. The protocol that would become the backbone of email as we know it. SMTP was simpler, more focused, and better suited to the emerging Internet architecture. The MPM's more complex relay model, with its multimedia ambitions and inter-network bridging, was quietly superseded.
Port 45 fell silent.
The Port Today
Port 45 remains officially assigned to MPM in the IANA Service Name and Transport Protocol Port Number Registry2. It sits in the System Ports range (0 through 1023), which means it was reserved by IANA for established services. That assignment has never been revoked.
No mainstream software uses port 45. It does not appear on common vulnerability lists. It has no documented association with known trojans or malware5. It is, for practical purposes, dormant.
If you find something listening on port 45, pay attention. There is no legitimate modern reason for a service to bind to this port. On Linux or macOS:
On Windows:
Any process listening on port 45 warrants investigation, because nothing should be there.
Why Ghost Ports Matter
The Internet has 65,535 ports. Only a fraction carry active, well-known services. But many of the "empty" ones are not truly empty. They are like port 45: assigned to protocols that mattered once, that solved problems that needed solving, that were superseded by something better.
These ghost ports are the Internet's archaeological record. Port 45 tells you that before SMTP, before the modern email system that carries billions of messages a day, someone built a message routing architecture and gave it a port number and made it work. The fact that it was replaced does not diminish what it was. It was the proof of concept for the idea that messages could flow between incompatible networks, routed by intelligent relay nodes, stored and forwarded until they reached their destination.
That idea did not die. It evolved into SMTP, into modern email infrastructure, into every message routing system that followed. Port 45 is where it started listening.
Related Ports
- Port 46 (MPM-SND): The companion port for MPM sending operations2
- Port 25 (SMTP): The protocol that succeeded MPM for email delivery
- Port 109 (POP2) and Port 110 (POP3): Later protocols for retrieving delivered messages
- Port 143 (IMAP): The modern standard for mailbox access
Frequently Asked Questions
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