BRev Port 143 — IMAP
IMAP keeps the mailbox on the server. Folders, read and unread state, flags, drafts and searches all live there, so a phone, a laptop and webmail show the same thing and a lost device loses nothing — the difference from POP3 that matters far more than any encryption detail.
Port 143 is the original, unencrypted IMAP port. Many servers do offer STARTTLS on it, which upgrades the connection in-band, and a client configured to require that upgrade is genuinely encrypted. The problem is “require”: a client set to prefer STARTTLS will silently fall back to cleartext if the advertisement is missing or stripped, and cleartext IMAP means the password and every message body are readable on the wire. Our reference marks 143 never for that reason.
Port 993 removes the ambiguity by negotiating TLS before any IMAP command. That is the port to configure. Port 143 still has a legitimate life inside a host — a local IMAP proxy or a webmail front end talking to a server on the same machine over loopback — where there is no network to eavesdrop on.
Port 143 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 143 | Well-known |
| Service | IMAP | Reads mail on the server, plain text. (TCP) |
| Category | Mail | sending and collecting email |
| Exposure | Never | Never expose to the internet |
Should port 143 face the internet? Cleartext credentials — publish IMAPS (993) instead.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
The client works on 143 and fails on 993
A connection-security mismatch. 993 needs the client set to SSL/TLS, not STARTTLS — a client sending IMAP commands to an implicit-TLS port simply hangs. It can also surface a certificate problem that 143 never checked: a self-signed certificate, or a name that does not match the server you typed.
You cannot tell whether the connection is actually encrypted
Configure the client to REQUIRE encryption and see whether it still connects — a client that succeeds with “required” is encrypted; one that only succeeds with “preferred” was probably falling back. On the server side, most IMAP daemons can refuse plaintext authentication outright, which is the setting worth having.
Check port 143 yourself
No browser can open a raw TCP socket, so no web page can honestly tell you whether this port is open on a remote host. These are the real commands, generated for port 143 — swap example.com for the host you are actually testing.
Is port 143 reachable on another host?
nc -vz example.com 143"succeeded!" means the TCP handshake completed — the port is open. "Connection refused" is closed; a hang until timeout means filtered (a firewall dropped it silently).
nmap -Pn -p 143 example.comThe only common tool that distinguishes open / closed / filtered. -Pn skips the ping sweep, so it still works against hosts that ignore ICMP.
Test-NetConnection example.com -Port 143Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 143A blank screen or a banner means it connected. "Connection refused" means closed. Quit with Ctrl-] then `quit`.
curl -v --max-time 5 telnet://example.com:143Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 143?
sudo ss -tlnp | grep :143Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :143The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :143The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
No web page can tell you whether a port is open on a remote host
A browser cannot open a raw TCP socket — there is no API for it, by design. So a site that reports “port 3389 is OPEN on 203.0.113.10” is not checking from your browser; it is asking its own server to scan on your behalf. That works, but it means their server sees the host you asked about, scans from their address rather than yours, and answers a question about their network path, not yours.
We don't have a server here and won't pretend to. So this page does three honest things instead:
| Port | Proto | Service | What it is | Exposure | |
|---|---|---|---|---|---|
| 143 | tcp | IMAP | Reads mail on the server, plain text.Cleartext credentials — publish IMAPS (993) instead. | Neverbrowser-blocked | |
| 1433 | tcp | Microsoft SQL Server | The MSSQL wire protocol.Never public — bind to localhost or a private subnet. It is brute-forced around the clock. | Never | |
| 1434 | udp | SQL Server Browser | Tells callers which port each SQL instance uses.Instance enumeration, and the Slammer worm’s amplifier. | Never | |
| 11434 | tcp | Ollama | Local LLM model server API.Free GPU for whoever finds it, and the model-pull API can write files. | Never |
http://host:port/ and times it. A connection that is established proves a socket is open; an instant refusal usually means nothing is listening; anything else is honestly reported as “can't tell”. Each attempt gives up after 2.5s.nc -vz example.com 443
nmap -Pn -p 443 example.com
telnet example.com 443
curl -v --max-time 5 telnet://example.com:443
This page cannot answer that, and neither can any other page — from your browser. Opening a TCP connection to an arbitrary host and port is not something JavaScript is allowed to do. Every “open port checker” that gives you a green tick is running the scan on their server. Which is fine, as long as you know what you are agreeing to: they learn which host and port you were worried about, and the answer they give you is about the route from their data centre — not from the internet as your users see it.
What to do instead
Run the check from outside your own network. Copy an nmap or nc line from the card above and run it from a machine that is not behind the same router — a phone on mobile data with a terminal app, a cheap VPS, a colleague's laptop. Running it from inside the LAN tests the LAN, not the internet, and will happily say “open” while the world sees nothing.
Then check the thing that is actually blocking it, in this order: the service is bound to0.0.0.0 and not 127.0.0.1 · the host firewall (ufw, firewalld, Windows Defender Firewall) · the cloud security group or the router's port-forward rule · and finally your ISP, which commonly blocks 25, 80 and 445 on residential lines whatever you configure.
What a browser genuinely can verify
One thing: that an HTTPS service answers. If what you actually want to know is “is my site up and how fast is it”, the latency test measures real round trips to a CORS-open URL. If the name doesn't resolve, or resolves somewhere unexpected, that is a DNS problem, not a port problem — and it is the more common cause of “my port is closed” than an actual firewall.
An IP address finds the machine; a port picks which program on it gets the traffic. TCP sets up a connection first — a handshake, ordered delivery, retransmission — which is why the web, email and SSH use it. UDP just sends the packet with no handshake and no guarantee it arrives, which is why DNS, video calls and games use it: late data is worse than lost data.
That difference is why a UDP port is so much harder to test. TCP either completes a handshake or it doesn't, so a tool can tell you. A UDP service that has nothing to say to an unexpected packet stays silent — and silence looks identical to a firewall dropping it.
The three ranges
0–1023Assigned by IANA to core services. On Unix, binding one needs root (or CAP_NET_BIND_SERVICE).1024–49151Registered by vendors for their applications. Any user can bind these — which is why every dev server lives here.49152–65535Never assigned. The OS hands these out as the SOURCE port of your outgoing connections. (Linux actually uses 32768–60999 by default.)Closed is not the same as filtered
Closed means the host answered: the packet arrived, nothing is listening, and you got an immediate refusal (a TCP RST). Filtered means nothing answered at all — a firewall dropped your packet in silence, so you wait for a timeout and learn nothing. The difference matters: closed proves the host is reachable and the port is free; filtered tells you only that something between you and it is discarding traffic.nmap reports the two separately. A browser cannot distinguish them, which is why this page reports a timeout as “can't tell” rather than guessing.
The one habit worth keeping
Bind services to 127.0.0.1 instead of 0.0.0.0 unless something outside the machine genuinely needs them, and reach them over SSH or a VPN. Every database on the never-expose list above ships with that advice, and the leaks all begin the same way: a default bind address, a cloud instance with a public IP, and a scanner that found it within the hour.
The blocked-port list this page uses is the WHATWG Fetch Standard's “bad port” table — the ports every browser refuses to connect to (Chrome calls it ERR_UNSAFE_PORT). Local probing is additionally gated by Chrome's Local Network Access permission, which replaced Private Network Access and ships in Chrome 142.
FAQ
Is port 143 encrypted?
Not by itself. It can be upgraded with STARTTLS, but the connection starts in the clear and falls back to cleartext if the upgrade is not enforced. Port 993 negotiates TLS first and has no such fallback.
IMAP or POP3?
IMAP for anything used from more than one device: the mailbox, folders and read state stay on the server and synchronise. POP3 downloads to a single client and, by default, deletes from the server.
Can I keep 143 open internally?
Yes, where the traffic never crosses a network you do not trust — a webmail front end or a local proxy talking to the IMAP server over loopback. Reaching it across any routed network should be 993.