Port 220 sits in the well-known port range (0-1023) with an official IANA assignment: IMAP3, Interactive Mail Access Protocol version 3. But there's a problem. IMAP3 has been dead since 1993.
What IMAP3 Was Supposed to Be
In February 1991, James Rice at Stanford published RFC 12031, defining IMAP3. It was written specifically as a counter-proposal to RFC 1176, which was updating IMAP2. Rice thought he had a better way forward for email access protocols.
The Internet thought otherwise.
Why IMAP3 Failed
IMAP3 was never accepted by the marketplace. When the IMAP Working Group sat down to plan the future of Internet email access, they looked at RFC 1203 (IMAP3) and RFC 1176 (IMAP2), and chose IMAP2 as their starting point.2
Two years after publication, the IESG reclassified RFC 1203 as a "Historic" protocol in 1993.3 That's the Internet's way of saying: "This exists in the archives, but nobody should use it."
IMAP2 became the foundation for IMAP4, which moved to port 143 and became the protocol that millions of people use today to check their email. Port 220 was left behind, officially assigned to a protocol that never saw real-world adoption.
What Actually Uses Port 220 Today
Officially: nothing. IMAP3 is historic, meaning obsolete.
Unofficially: some network administrators configure SSH (Secure Shell) to run on port 220 as an alternative to the standard port 22.4 This is usually done as a security measure—running SSH on a non-standard port can help avoid automated attacks that target port 22.
This represents the common reality of port assignments: when an official assignment goes unused, unofficial uses fill the void.
Why This Port Still Matters
Port 220 is a reminder that not every protocol succeeds. The well-known port range (0-1023) contains both the essential infrastructure of the Internet and the ghosts of protocols that didn't make it.
IANA doesn't typically reassign well-known ports even when their original purpose disappears. The assignment remains as historical documentation. Port 220 tells the story of IMAP3: a protocol that competed, lost, and was marked historic within two years.
How to Check What's Using Port 220
On Linux or macOS:
On Windows:
If you see something listening on port 220, it's not IMAP3. It's likely SSH or another service that borrowed an unused port assignment.
The Well-Known Port Range
Port 220 belongs to the System Ports or Well-Known Ports range (0-1023). These ports are assigned by IANA through the "IETF Review" or "IESG Approval" procedures—formal processes that involve documentation and consensus.5
Getting a well-known port assignment in 1991 meant something. It meant your protocol was important enough to reserve a permanent place in the Internet's address space. But assignment didn't guarantee adoption.
Security Considerations
The official IMAP3 protocol on port 220 had no built-in encryption.6 This was common for early email protocols—security was added later through separate SSL/TLS layers.
If you encounter port 220 in use today, determine what's actually running on it. If it's SSH, ensure you're using key-based authentication and keep the software updated. If it's something unexpected, investigate.
Related Ports
- Port 143: IMAP4, the successor that actually succeeded
- Port 993: IMAPS (IMAP over SSL/TLS)
- Port 22: SSH's standard port (often the reason SSH shows up on 220)
Frequently Asked Questions
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