Port 406 carries IMSP (Internet Message Support Protocol), an experimental companion to IMAP4 that was designed to solve problems IMAP couldn't handle alone—and then was abandoned before ever becoming an official standard.
The strange part: it's still running in production. Over 1,000 sites. More than a million users. Two decades after development ceased.1
What IMSP Does
IMSP was designed as a companion to IMAP4, handling the parts of email infrastructure that fell outside the scope of mail access itself:2
- Shared mailbox management across multiple IMAP servers
- Configuration options that follow users between servers
- Address books accessible from any client
The insight: in medium-to-large email operations, you don't run one IMAP server. You run many. And when a user moves between servers, their preferences and address books shouldn't vanish. IMSP was the glue that made multi-server email domains feel like a unified system.
The Carnegie Mellon Story
John G. Myers at Carnegie Mellon University authored the IMSP specification in March 1994.3 Carnegie Mellon had deep experience with email infrastructure—this was the institution behind the Andrew Project, which produced groundbreaking work in distributed computing and messaging systems.4
IMSP ran on port 406 for both TCP and UDP, officially registered with IANA.5
But IMSP never became an RFC. It remained an Internet Draft, with status "experimental."
Why It Was Abandoned
In 1995, Myers and his colleagues realized something: the problem IMSP solved—storing and synchronizing user preferences—wasn't unique to email. Every Internet application needed a way to store client configuration that followed users across servers.2
So they stopped working on IMSP and started building ACAP (Application Configuration Access Protocol)—a broader protocol that could handle preferences for any application, not just email.
IMSP was too narrow. ACAP would be universal.
The Twist
ACAP never took off.
IMSP, the "failed" experimental protocol that was abandoned mid-development, kept running. By the 2000s, it was serving over a million users.2
This happens sometimes in Internet infrastructure. The thing that was supposed to fail keeps working. The thing that was supposed to replace it disappears. And nobody has the incentive to migrate away from something that works, even if it was never meant to be permanent.
Security Considerations
IMSP predates modern security practices. As an experimental protocol from 1994, it was designed in an era when encryption was optional and authentication was simpler.
Some security resources note that port 406 has been used by malicious software in the past,6 though this is true of many lesser-known ports—attackers use whatever's available.
If you find IMSP running on your network:
- Verify it's legitimate (are you actually running a multi-server IMAP environment with IMSP?)
- Ensure it's not exposed to the public Internet
- Consider whether modern alternatives (like LDAP for address books, or centralized configuration management) might be more secure
How to Check What's on Port 406
On Linux or macOS, use netstat to see what's listening:
Or use lsof for more detail:
On Windows, use netstat with different flags:
If you see something listening on port 406, check the process name. If it's not IMSP and you don't recognize it, investigate further.7
Related Ports
- Port 143 - IMAP (the protocol IMPM was designed to complement)
- Port 993 - IMAPS (IMAP over TLS/SSL)
- Port 389 - LDAP (often used for address book functionality today)
Why This Port Matters
Port 406 represents a particular kind of Internet infrastructure: the experimental protocol that became accidentally permanent. Built as a temporary solution, abandoned for something better, yet still running in production because migrating away is harder than keeping it alive.
The Internet is full of these fossils—protocols that were never meant to last but became load-bearing because they worked well enough and replacing them was never urgent enough.
IMSP is one of them. Experimental. Superseded. Still carrying email preferences for a million people.
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