Port 2390: RSMTP — The Email Protocol That Almost Was

What This Port Is

Port 2390 sits in the registered port range (1024–49151). These are ports that applications and services can register with IANA, the organization that manages Internet number assignments. A registration signals intent and prevents collisions — it doesn't guarantee the service ever shipped.

Port 2390 was registered for RSMTP: Really Simple Mail Transport Protocol.

The Protocol That Wanted to Replace SMTP

In January 2000, four engineers filed a patent for a new way to move email between servers. Their argument was straightforward: SMTP, the protocol that has carried Internet email since 1982, was designed for a simpler world and was showing its age.

Their specific complaints were legitimate:

SMTP only speaks 7-bit ASCII. Email wasn't originally built for binary attachments or non-English characters. MIME encoding was bolted on later as a workaround — your PDF attachment gets inflated by roughly 33% to fit through a protocol designed for plain text.

SMTP wastes round trips. Every message involves a back-and-forth negotiation: greeting, sender declaration, recipient declaration, data transfer, confirmation. RSMTP was designed to reduce this stop-and-wait overhead.

SMTP requires dot stuffing. If a message body contains a line that's just a period, SMTP requires doubling it (so it becomes two periods) to prevent confusion with the end-of-message signal. Servers on both ends transform the message. It's a small indignity that has persisted for decades.

RSMTP proposed handling binary data natively, cutting unnecessary exchanges, and dropping the archaic per-byte processing requirements. The patent was granted in September 2006 as US7103635B2.¹

What Happened

Nothing. SMTP and its extension family (ESMTP, STARTTLS, and others) continued to dominate email infrastructure. Replacing a protocol that every mail server on the Internet speaks requires simultaneous adoption everywhere — a coordination problem that's effectively insurmountable without a mandate.

RSMTP is a clean example of a technically sound idea that lost to installed base. The port number exists. The patent exists. The running servers do not.

Security Notes

Some port databases flag 2390 as having been associated with malware traffic historically.² This is common for any unmonitored port in the registered range — when legitimate services don't occupy a port, malicious software sometimes does. If you see unexpected traffic on 2390, investigate it.

Checking What's on This Port

If port 2390 is open on a machine you administer, something put it there. Check with:

Linux/macOS:

ss -tulnp | grep 2390
# or
lsof -i :2390

Windows:

netstat -ano | findstr :2390

Then use the process ID from those results to identify the application. On Linux: ps aux | grep <PID>. On Windows: Task Manager → Details tab.

Why Unassigned (and Abandoned) Ports Matter

The registered port range contains thousands of ports like 2390: claimed for a specific purpose, never widely deployed, now sitting quiet. This matters for a few reasons:

Firewall hygiene. Ports that should have no traffic are useful sentinels. Unexpected activity on 2390 is more suspicious than activity on 443.

Application defaults. Software developers sometimes pick registered-but-unused ports for internal services, assuming they're safe. That assumption holds until it doesn't.

The port isn't "empty." It has a name, a history, and a patent. That context is useful when diagnosing why traffic appears there.