Port 366 carries ODMR (On-Demand Mail Relay), a protocol created to solve a problem from the dial-up era that most people solved by simply not running their own email servers.
What ODMR Does
ODMR is an extension of SMTP designed for systems with dynamic IP addresses—the kind you get when you dial in to an ISP and receive a different address every time.1
In the late 1990s, if you wanted to run your own email server from home on a dial-up connection, you faced a fundamental problem: your IP address kept changing. SMTP requires stable addresses. Mail servers need to know where to send your email. DNS records need to point somewhere permanent.
ODMR worked like this:2
- You (the customer with the dynamic IP) initiate a connection to your provider's mail server on port 366
- You authenticate yourself
- The roles reverse—suddenly your machine becomes the server, their machine becomes the client
- They deliver all your pending mail using normal SMTP
- Connection ends, everyone goes home
It uses a command called ATRN (Authenticated TURN), which is like saying "I'm ready to receive mail now, and here's proof I'm allowed to."3
The Problem It Solved
In 1999, Randall Gellens at Qualcomm saw a specific problem: people wanted to run their own mail servers, but dial-up gave them a new IP address every session.4
Regular mail servers couldn't handle this. If your DNS record said your mail server was at 203.45.67.89 but you dialed in and got 203.45.67.142, incoming mail would fail. You'd never receive anything.
The existing solutions were clumsy. POP and IMAP meant storing mail on someone else's server. UUCP was old and limited. The ETRN command existed, but it didn't work for dynamic IPs.
ODMR solved this by letting your mail server call out to fetch mail, authenticate on demand, and receive everything waiting for you—all on port 366.
Why It Disappeared
RFC 2645 standardized ODMR in August 1999.5 It was declared obsolete on November 20, 2008.6
Nine years. That's how long it took for the problem ODMR solved to become irrelevant.
What happened? Two things:
First, broadband replaced dial-up. Static IPs became cheap. Dynamic DNS services solved the addressing problem. The entire premise of "I have a different IP every time I connect" became less common.
Second—and more important—people stopped running their own mail servers from home. Gmail launched in 2004. Spam filtering became impossible for individuals to maintain. ISPs started blocking port 25 from residential IPs. Running your own mail server went from "difficult but possible" to "why would you even try?"7
ODMR solved a real problem. But the world moved past that problem entirely. We didn't adopt ODMR widely—we just stopped doing the thing that needed ODMR in the first place.
What's Genuinely Strange
Port 366 represents a specific kind of obsolete: it was assigned, standardized, and abandoned before most people even knew it existed.
Look at port 25 (SMTP) or port 110 (POP3)—those ports carried millions of connections for decades. Port 366? It solved a problem during a narrow window when:
- People wanted to run their own mail servers (declining)
- From dial-up connections (disappearing)
- Without static IPs (becoming cheap)
- Before giving up entirely (inevitable)
That window lasted maybe five years. The protocol specification lasted nine.
Now port 366 sits mostly quiet. Not because ODMR failed—it worked fine. But because the entire premise vanished. The Internet moved on.
Security Notes
ODMR requires authentication before reversing roles—you can't just connect and receive mail.8 The protocol was designed with security in mind.
But here's the thing: almost nobody is using ODMR today. If you see traffic on port 366, it's either:
- A legacy system from the early 2000s still running
- Someone testing historical protocols
- A misconfigured service using the wrong port
If you're running ODMR in 2026, you're probably doing something very specific or very strange.
How to Check What's on Port 366
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. Port 366 is assigned, standardized, and empty.
Related Ports
- Port 25 — SMTP, the protocol ODMR extends
- Port 110 — POP3, the way most people actually retrieved mail
- Port 143 — IMAP, the better way to retrieve mail
- Port 587 — SMTP submission, what actually replaced most of this
Why This Port Matters
Port 366 matters not because it's widely used—it isn't. It matters because it shows how the Internet evolves.
We don't always solve problems by adopting new protocols. Sometimes we solve them by making the problem itself obsolete. ODMR was a good solution to dial-up email. Broadband and Gmail were better solutions to the entire concept of personal mail servers.
Port 366 is a monument to a problem we fixed by walking away from it entirely.
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