BRev Port 993 — IMAPS
Port 993 is IMAP inside TLS, with the handshake completed before any IMAP command is sent. It is how essentially every modern mail client reads mail, and it is one of the small set of entries in this reference marked safe to expose — it was designed to take unsolicited connections from the open internet and it authenticates and encrypts them properly.
One behaviour distinguishes it operationally: IMAP IDLE. Rather than polling, a client tells the server it is idle and the connection stays open until something changes, which is what makes new mail appear instantly. That means long-lived TCP connections — often one per open folder, per device — so aggressive firewall or NAT idle timeouts cause a mailbox that “stops updating until you pull to refresh”. The fix is at the network, not in the client.
The exposure verdict still assumes maintenance. It is a service parsing untrusted input from anyone on the internet, so the daemon needs patching, the TLS configuration needs to stay current, and the accounts behind it need rate limiting — credential stuffing against IMAP is a standard way into a mailbox, and legacy protocols are a common bypass for multi-factor policies.
Port 993 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 993 | Well-known |
| Service | IMAPS | IMAP over TLS — how mail clients read mail. (TCP) |
| Category | Mail | sending and collecting email |
| Exposure | Safe | Safe to expose |
Should port 993 face the internet? Designed for the open internet; encrypted and authenticated.
Built to face the internet: authenticated, encrypted, and expected to take unsolicited connections. Still patch it.
What commonly goes wrong
Mail only arrives when you manually refresh
IDLE is being cut. The client holds an open connection waiting for the server to push a change, and a NAT or firewall idle timeout shorter than the IMAP keepalive silently drops it. Raise the timeout on the network device, or configure the client to re-issue IDLE more frequently. Mobile clients often fall back to polling on battery-saver settings too.
Certificate errors that 143 never showed
Because 143 was not validating one. A self-signed certificate, a missing intermediate, or a certificate issued for mail.example.com while the client connects to imap.example.com all appear at this point. openssl s_client -connect host:993 -servername host prints the chain and the verification result.
Check port 993 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 993 — swap example.com for the host you are actually testing.
Is port 993 reachable on another host?
nc -vz example.com 993"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 993 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 993Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 993A 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:993Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 993?
sudo ss -tlnp | grep :993Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :993The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :993The 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 | |
|---|---|---|---|---|---|
| 993 | tcp | IMAPS | IMAP over TLS — how mail clients read mail.Designed for the open internet; encrypted and authenticated. | Safebrowser-blocked | |
| 16993 | tcp | Intel AMT (TLS) | AMT over TLS.Same hardware-level control, encrypted. | 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 993 safe to expose?
Yes — it is an internet-facing service by design, encrypted and authenticated. Keep the IMAP daemon patched, keep TLS current, and rate-limit authentication, because mailboxes are a standard credential-stuffing target.
What is the difference between 993 and 143?
993 establishes TLS before any IMAP command. 143 starts in the clear and may upgrade with STARTTLS, which can be stripped if the client does not require it. Same protocol, different guarantee.
Why does my mailbox hold several connections open?
IMAP IDLE keeps a connection per watched folder so the server can push changes instantly. Some servers cap concurrent connections per account, which is why a client with many folders occasionally reports “too many connections”.