BRev Port 110 — POP3
POP3 downloads mail to one device and, in its default configuration, deletes it from the server. That model made sense when storage was expensive and people had one computer; today it is mostly a liability, because the mail exists in exactly one place — the machine that fetched it — and no other device ever sees it. IMAP on 143/993 keeps the mailbox on the server and synchronises, which is what almost everyone actually wants.
On port 110 specifically, authentication is in the clear. USER and PASS are sent as plain text, readable by anyone on the network path, which is why our reference marks it never rather than care. The encrypted equivalent is POP3S on port 995, which is the same protocol inside TLS and is marked safe to expose.
The setting that causes real damage is “leave a copy on the server”. Turn it off and a laptop that fetches mail becomes the only copy, so a lost machine is lost mail; turn it on without a retention policy and the mailbox grows until the quota rejects incoming messages. Neither failure mode exists with IMAP, which is the stronger argument for migrating than the port number is.
Port 110 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 110 | Well-known |
| Service | POP3 | Downloads mail to a client, plain text. (TCP) |
| Category | Mail | sending and collecting email |
| Exposure | Never | Never expose to the internet |
Should port 110 face the internet? Passwords in the clear. Offer POP3S (995) instead.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
Mail disappeared from the webmail after setting up a phone or laptop
A POP3 client fetched and deleted it. The messages are on that device, not in the account — check its local mail store before assuming loss. Switch the account to IMAP so the server keeps the canonical mailbox and every device shows the same thing.
The provider refuses the connection on 110
Most providers have disabled cleartext POP3 entirely, or accept the connection and then refuse AUTH until TLS is negotiated. Move the client to 995 with SSL/TLS. If the provider offers neither, it is not enforcing encryption for mail credentials at all, which is worth escalating.
Check port 110 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 110 — swap example.com for the host you are actually testing.
Is port 110 reachable on another host?
nc -vz example.com 110"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 110 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 110Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 110A 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:110Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 110?
sudo ss -tlnp | grep :110Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :110The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :110The 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 | |
|---|---|---|---|---|---|
| 110 | tcp | POP3 | Downloads mail to a client, plain text.Passwords in the clear. Offer POP3S (995) instead. | Neverbrowser-blocked |
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 110 secure?
No. Credentials and message content are transmitted in plain text, so anyone in the network path can read both. Use POP3S on 995, or better, IMAP over TLS on 993.
POP3 or IMAP?
IMAP for almost every modern use: the mailbox stays on the server, folders and read state sync, and several devices can share the account. POP3 suits exactly one case — deliberately pulling mail down to a single archive machine and freeing server storage.
What is the encrypted version of port 110?
Port 995, POP3S — the same protocol inside TLS from the first byte. Some servers also support STARTTLS on 110, but 995 is the unambiguous choice.