SysAdmin Tools

An SMTP checker tests whether a mail server is reachable on the ports it uses to send and receive email. SMTP — the Simple Mail Transfer Protocol — is the language mail servers speak to move messages across the internet, and it listens on a small set of well-known ports: 25, 465, 587 and the alternative 2525. If any of the ports your mail flow depends on is blocked by a firewall, an ISP, or a cloud security group, email silently fails to send. This tool checks those ports for you from an external perspective, in parallel, and tells you exactly which are open and which are blocked.

Email delivery problems are among the hardest to diagnose because there is rarely a clear error — messages just sit in a queue or vanish. The most common root cause is a port that is not accessible. Port 25 is blocked by default on almost every cloud provider and consumer ISP to curb spam. Port 587 (submission) is the modern standard for authenticated sending and must be reachable for most mail clients and applications to relay mail. Port 465 (SMTPS) carries SMTP over an implicit SSL/TLS connection, and port 2525 is a widely supported fallback when the others are filtered.

This free SMTP checker attempts a TCP connection to each selected port on your domain or mail server hostname and reports the status and response time, alongside the domain's MX records so you can confirm you are testing the right server. It is the fastest first step when email is not sending — before you dig into authentication, DNS, or relay configuration, confirm the ports are actually open. For deeper credential and STARTTLS testing you still need command-line access to the server, but port accessibility is where most SMTP problems begin and end.

How to Use the SMTP Checker

  1. 1

    Enter your domain or mail server hostname

    Type the domain or the specific mail server you want to test — for example, mail.example.com or smtp.gmail.com. The tool strips http://, www. and any path automatically, then resolves the hostname before checking connectivity.

  2. 2

    Select which SMTP ports to check

    The four core SMTP ports (25, 465, 587 and 2525) are selected by default. Tick or untick any port, and optionally add the IMAP SSL (993) and POP3 SSL (995) mailbox-retrieval ports if you also want to verify inbound mail access.

  3. 3

    Click Check SMTP Server

    The tool tests every selected port in parallel from our servers and simultaneously looks up the domain's MX records. Results typically return within a few seconds, showing the status and response time for each port.

  4. 4

    Review port status and fix any blocked ports

    Read the results table and the summary verdict. If port 587 is open, authenticated sending should work. If a required port is blocked, adjust the relevant firewall, security group, or ISP setting — and remember port 25 being blocked is normal on most cloud servers.

Understanding SMTP Ports and Results

SMTP uses several ports, each with a distinct role, and knowing which should be open is the key to reading the results. Port 587 is the submission port — the modern standard for a mail client or application to hand a message to a server using SMTP authentication and STARTTLS. This is the port that matters most for sending email, and it should be open. Port 465 carries SMTP over an implicit SSL/TLS connection (SMTPS); it is widely supported and a common alternative to 587. Port 25 is the original server-to-server relay port. It is almost always blocked outbound by cloud providers and ISPs to prevent spam, so a closed port 25 is expected and usually not a problem for sending through a provider. Port 2525 is not an official SMTP port but is offered by many email delivery services (SendGrid, Mailgun, Mailjet and others) as a fallback when 587 and 465 are filtered. A result of Open means the TCP connection succeeded and something is listening; Closed means the connection was refused or timed out — either nothing is listening or a firewall is blocking it. Because this tool checks reachability from the public internet, it shows what an external client actually sees, which is exactly the perspective that matters for email delivery.
FieldDescription
Port 25 (SMTP)The classic server-to-server mail relay port. Blocked outbound by default on almost all ISPs and cloud providers to fight spam — a closed result here is normal.
Port 465 (SMTPS)SMTP wrapped in an implicit SSL/TLS connection. Widely used for authenticated sending as an alternative to port 587.
Port 587 (Submission)The recommended modern port for authenticated mail submission using STARTTLS. This is the port that should be open for sending email.
Port 2525 (Alt SMTP)An unofficial fallback port offered by many email delivery services when 587 and 465 are blocked by the network.
Status: OpenThe TCP connection succeeded — a mail service is listening on this port and is reachable from the public internet.
Status: ClosedThe connection was refused or timed out. Nothing is listening on this port, or a firewall or security group is blocking it.

Advertisement

When to Use the SMTP Checker

Diagnosing why an application cannot send email

When a web app, CRM, or script fails to send mail, the first thing to confirm is that the SMTP port it uses (usually 587 or 465) is actually reachable. A blocked port is the single most common cause and this tool confirms it in seconds.

Verifying a new mail server after setup

After configuring Postfix, a mail appliance, or a cloud email service, check that the expected submission and SMTPS ports are open from the outside. This catches missing firewall or security-group rules before real email starts bouncing.

Confirming whether port 25 is blocked

If you are trying to run your own outbound mail relay from a cloud VM, check port 25. Most providers block it by default, so a closed result tells you to request an unblock or switch to a relay on port 587 or a smart host.

Choosing a fallback port for an email service

When a restrictive network blocks the standard SMTP ports, test port 2525 to confirm a delivery service like SendGrid or Mailgun will still be reachable, then configure your application to use it as a fallback.

SMTP Checker — Frequently Asked Questions

What is SMTP and how does it work?
SMTP (Simple Mail Transfer Protocol) is the internet standard for sending email between servers. When you send a message, your mail client submits it to an SMTP server, which then relays it from server to server until it reaches the recipient's mailbox provider. Each step opens a TCP connection on an SMTP port, exchanges commands such as EHLO, MAIL FROM and RCPT TO, and transfers the message. SMTP only handles sending and relaying; retrieving mail from a mailbox uses IMAP or POP3 instead.
What ports does SMTP use?
SMTP uses four main ports. Port 25 is the original server-to-server relay port. Port 587 is the submission port for authenticated sending with STARTTLS and is the modern recommended choice for mail clients. Port 465 carries SMTP over an implicit SSL/TLS connection (SMTPS). Port 2525 is an unofficial alternative offered by many email delivery services as a fallback when the standard ports are blocked. Mailbox retrieval uses different ports: 993 for IMAP SSL and 995 for POP3 SSL.
How to test if SMTP port is open?
Enter your mail server hostname in the SMTP checker above, select the ports you want to test, and click Check SMTP Server. The tool attempts a TCP connection to each port from our servers and reports whether it is open or blocked, along with the response time. From a command line you can achieve the same with telnet mail.example.com 587 or openssl s_client -connect mail.example.com:465, but the online tool tests from an external perspective without any setup.
What is the difference between port 25, 465 and 587?
Port 25 is for server-to-server relaying and is blocked outbound by most ISPs and cloud providers to prevent spam. Port 587 is the submission port: mail clients and applications use it to send authenticated email with STARTTLS, upgrading the connection to encryption after connecting. Port 465 is SMTPS, where the connection is encrypted with SSL/TLS from the very start (implicit TLS). For sending email today, use port 587 or 465 with authentication; reserve port 25 for inbound server relay.
Why is port 25 blocked on most servers?
Port 25 is blocked outbound by default on nearly all cloud providers (AWS, Google Cloud, Azure) and consumer ISPs to stop compromised machines and spammers from sending bulk unsolicited email directly. Because port 25 requires no authentication for server-to-server relay, an open port 25 is a favourite target for abuse. If you need to send outbound mail from a cloud server, you generally request an unblock through the provider's abuse team or, more commonly, relay through an authenticated service on port 587 or a smart host.
How to check SMTP server connectivity?
The quickest way is to test the SMTP ports for accessibility, which is exactly what this tool does: it opens a TCP connection to ports 25, 465, 587 and 2525 on your mail server and reports which respond. Confirm that the submission port your application uses is open, check the domain's MX records point to the right server, and verify response times are reasonable. For a full end-to-end test including authentication and TLS negotiation, use openssl or telnet directly against the server.
What is SMTP authentication?
SMTP authentication (SMTP AUTH) is the mechanism a mail client or application uses to prove its identity to a submission server before sending email. After connecting on port 587 or 465 and securing the connection with TLS, the client supplies a username and password using a method such as AUTH LOGIN or AUTH PLAIN. Only then is it allowed to relay mail. Authentication prevents open relaying — where anyone could send mail through the server — and ties outgoing messages to a known account, which is essential for deliverability and abuse prevention.
How to fix SMTP connection refused error?
A "connection refused" error means nothing is listening on the port or a firewall is rejecting it. First confirm the port is actually open using this SMTP checker. Then check, in order: that the mail service is running and bound to the port, that the OS firewall (ufw, iptables, Windows Firewall) allows it, that the cloud security group has an inbound rule, and that you are using the correct port — 587 for submission rather than a blocked port 25. Correct the layer that is blocking and re-test.
What is STARTTLS in SMTP?
STARTTLS is a command that upgrades a plain-text SMTP connection to an encrypted one. The client connects normally on port 587 (or 25), issues the STARTTLS command, and the two sides then negotiate a TLS session so that the rest of the exchange — including authentication credentials and the message — is encrypted. This is called explicit TLS, in contrast to the implicit TLS used on port 465, where encryption is active from the moment the connection opens. STARTTLS lets a single port support both encrypted and unencrypted sessions.
How to test email server without sending email?
You can verify an email server without sending a message by testing port accessibility and inspecting its greeting. This SMTP checker connects to the mail server's ports and confirms they are open without ever transmitting an email. From a command line, telnet mail.example.com 587 or openssl s_client -connect mail.example.com:465 opens a session where the server responds with its 220 banner and you can issue EHLO to see supported features — all without submitting a message. This is the safe way to validate connectivity and configuration.

Related Tools

Email Health Checker

Audit MX, SPF, DKIM, DMARC and blacklist status for the whole domain.

Port Checker

Test any single TCP port on any host, not just SMTP ports.

MX Lookup

Check the mail exchange records and priorities for a domain in detail.

Email Header Analyzer

Trace an email's delivery path and authentication results from its headers.