BRev Port 587 — SMTP submission
Port 587 is where a mail client hands an outgoing message to its own server. RFC 6409 defines it as message submission and separates it deliberately from port 25’s server-to-server transfer, because the two need different rules: submission requires authentication and may rewrite the message (adding a Message-ID, fixing headers, signing with DKIM), while transfer accepts mail from strangers and must not.
That separation is also what makes it work from anywhere. ISPs and cloud providers block outbound 25 to contain spam from compromised machines, and they deliberately leave 587 alone — so a laptop on hotel Wi-Fi can submit mail while it could never talk to port 25. If a mail client fails to send away from the office, this is almost always the fix.
The encryption model is STARTTLS: the connection opens in the clear and is upgraded in-band. That works, but it can be stripped by an active attacker who removes the STARTTLS advertisement, so the client must be configured to require TLS rather than to prefer it — “STARTTLS, required” in the client, not “if available”. Port 465 avoids the problem by starting TLS before any SMTP is spoken; both are current and correct.
Port 587 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 587 | Well-known |
| Service | SMTP submission | Where mail clients hand outgoing mail to your server. (TCP) |
| Category | Mail | sending and collecting email |
| Exposure | Care | Expose with care |
Should port 587 face the internet? Must be public for roaming users — require STARTTLS and authentication, never relay for strangers.
Legitimately public in some setups, but only hardened — current version, real authentication, rate limits, and ideally an allow-list.
What commonly goes wrong
“Authentication failed” only on 587
Many servers refuse AUTH before the connection is encrypted, so a client that has not completed STARTTLS is told authentication is unavailable — which surfaces as a credential error. Set the client to require STARTTLS on 587 and re-test. On providers with two-factor authentication, an app-specific password is usually required as well.
Mail is sent but lands in spam
Nothing to do with the port. Delivery depends on SPF authorising the sending host, DKIM signing surviving intact, and DMARC alignment between the visible From domain and one of those. Check all three together rather than tuning the mail server — the email DNS checker linked below runs the whole set.
Check port 587 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 587 — swap example.com for the host you are actually testing.
Is port 587 reachable on another host?
nc -vz example.com 587"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 587 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 587Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 587A 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:587Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 587?
sudo ss -tlnp | grep :587Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :587The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :587The 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 | |
|---|---|---|---|---|---|
| 587 | tcp | SMTP submission | Where mail clients hand outgoing mail to your server.Must be public for roaming users — require STARTTLS and authentication, never relay for strangers. | Carebrowser-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
Port 587 or 465?
Both are correct today. 587 opens in the clear and upgrades with STARTTLS; 465 is TLS from the first byte (implicit TLS), which cannot be downgrade-stripped. Use whichever your provider documents, and require TLS either way.
Why does my ISP block 25 but not 587?
Because 587 requires authentication, so a compromised machine cannot use it to spam anonymously — it needs valid credentials for a specific account, which is traceable and revocable. Port 25 accepts anonymous relay attempts, which is what made it worth blocking.
Does port 587 need authentication?
Yes, by definition. A submission port that accepts unauthenticated mail is an open relay with a different number on it, and it will be abused just as quickly.