Port 783 occupies a peculiar space in the Internet's infrastructure. Officially, it's unassigned. IANA's registry lists ports 781-785 as having no designated service. But ask any mail server administrator what runs on port 783, and they'll tell you: SpamAssassin.1
The Well-Known Range
Port 783 sits in the well-known port range (0-1023), the space originally reserved for services assigned by IANA. These ports require root privileges to bind on Unix-like systems. They're supposed to represent standardized services with official assignments.
Port 783 has neither standardization nor assignment. It's an empty slot that got occupied anyway.
SpamAssassin's Claim
SpamAssassin is an email spam filtering system that uses content analysis, Bayesian filtering, and collaborative filtering databases to identify spam. The spamd daemon—the server component—traditionally listens on port 783.2
There's no RFC defining this. No official IANA assignment. Just a default configuration that spread across thousands of mail servers and became reality through deployment rather than specification.
The spamd daemon doesn't require port 783. It can listen on any port the administrator configures. But 783 became the conventional choice, documented in configuration guides and expected by administrators.
What This Port Means
When you see port 783 listening on a server, it's almost certainly SpamAssassin's spamd. The daemon accepts connections from mail transfer agents, analyzes email content, and returns spam scores.
But this is convention, not standard. Another service could legitimately use port 783. The port is unassigned—technically available to anyone. SpamAssassin just got there first and stayed.
The Gap Between Official and Actual
Port 783 represents something common in Internet infrastructure: the gap between official assignments and actual practice. IANA maintains the registry. Administrators deploy what works. Sometimes these align. Sometimes a port just gets claimed through use.
This creates a curious situation. Port 783 appears in security scanning tools, firewall configurations, and network documentation as "the SpamAssassin port." But open IANA's official registry, and it's listed as unassigned.
Both are true. The official record says one thing. The Internet does another.
Security Considerations
If you see port 783 open on a server you don't administer, it likely means SpamAssassin is running. But "likely" isn't "certainly." Because the port has no official assignment, anything could be listening there.
The spamd daemon itself has had security vulnerabilities over the years. An exposed spamd instance could be exploited if not properly configured. Many administrators run spamd on localhost only, avoiding external exposure entirely.3
But when port 783 is exposed externally, it's declaring: "I filter spam here." That's useful information for an attacker.
Checking What's Listening
On Linux or macOS:
On Windows:
If something is listening, these commands show what process owns the port. On a mail server, it's probably spamd. On any other system, port 783 shouldn't be listening at all.
Why Unassigned Ports Matter
The well-known range contains 1,024 ports. Not all are assigned. The gaps represent space for future standardization—or space for services like SpamAssassin that need a conventional port without going through formal assignment.
Port 783 works precisely because it's unassigned. SpamAssassin could pick it without conflicting with an official service. The lack of assignment became the reason for the assignment.
This is how the Internet grows. Not always through standards bodies and formal process. Sometimes through software that ships with a default, administrators who accept that default, and convention that hardens into infrastructure.
Port 783 is unassigned. Until you run a mail server. Then it's SpamAssassin's port. Both things are true, and the gap between them is where the actual Internet lives.
بۇ بەت پايدىلىق بولدىمۇ؟