Port 592: Eudora Set — The port that was registered but never used

Port: 592
Service: eudora-set (Eudora Set)
Protocols: TCP, UDP
Status: Officially assigned, apparently unused

Port 592 tells a story about intention versus reality. It was officially registered with IANA for a service called "eudora-set," assigned to Randall Gellens at Qualcomm—the company behind Eudora, one of the most dominant email clients of the 1990s.¹²

But here's the strange part: there's almost no evidence this protocol was ever actually used.

What Is Eudora Set?

The IANA registry lists port 592 as assigned to "eudora-set" for both TCP and UDP.³ The name suggests it was meant for configuring or managing settings in the Eudora email client.

Eudora was known for its extensive customization options—many accessible through special x-eudora-setting URIs rather than the standard interface.⁴ It's possible that port 592 was reserved for a remote configuration protocol, a way to push settings to Eudora clients across a network.

But if that protocol existed, it left almost no trace. No documentation. No implementation details. No users asking how to configure it.

The Eudora That Was

To understand why this port exists, you need to understand what Eudora was.

In 1995, Qualcomm claimed 64.7% of all email software revenue.⁴ By 1996, Eudora had 18 million users—the most widely used Internet email software in the world.⁴ Created by Steve Dorner at the University of Illinois in the 1980s and acquired by Qualcomm in 1991, Eudora was the email client before Outlook and Gmail took over.⁵

It was feature-rich, customizable, and powerful. Qualcomm clearly had plans for it—including registering port 592 for "eudora-set."

But the protocol never materialized. Eudora continued using standard email ports: 110 for POP, 143 for IMAP, 25 and 587 for SMTP.⁶ Port 592 remained assigned but silent.

Why Unassigned-in-Practice Ports Matter

Port 592 sits in the well-known port range (0-1023), which requires IANA assignment.⁷ These ports are supposed to be reserved for standardized, widely-used services—HTTP on 80, HTTPS on 443, SSH on 22.

But the registry is also full of ports like 592: officially assigned, technically reserved, practically forgotten.

This matters because:

• The namespace is finite — There are only 1,024 well-known ports. Each one reserved for a service that never shipped is a slot that can't be reused.
• It reveals abandoned plans — Every assigned port represents someone's intention. Port 592 is evidence that Qualcomm once planned something for Eudora that never happened.
• It's technically still claimed — You could run a service on port 592, but you'd be squatting on Qualcomm's official assignment. Most firewall rules and network tools still list it as "eudora-set."

What Runs on Port 592 Today?

Probably nothing.

You can check what's listening on port 592 on your system:

# On Linux or macOS
sudo lsof -i :592
sudo netstat -tulpn | grep :592

# On Windows
netstat -ano | findstr :592

If something appears, it's either:

• A legacy Eudora installation (unlikely unless you're running a very old system)
• A custom application someone built that happens to use port 592
• Malware trying to hide in an obscure port assignment

Most likely, you'll see nothing. Port 592 is assigned but empty—a placeholder for a protocol that never arrived.

The Ghost in the Registry

Port 592 is a reminder that the IANA port registry isn't just a list of what exists—it's also a record of what was planned.

Someone at Qualcomm, probably in the mid-1990s when Eudora dominated email, thought "we're going to need a port for remote configuration." They registered it. They gave it a name. And then... nothing.

The port remains assigned. The protocol never shipped. Eudora itself is now open source and effectively abandoned, its source code preserved by the Computer History Museum.⁸

Port 592 is a ghost—officially present, functionally absent, a small monument to a feature that never was.