What SMTP port should I use to send email?

Use port 587 with STARTTLS for all outbound email submission from applications and mail clients. This is the current standard. Port 465 with implicit TLS is also acceptable. Port 25 is reserved for server-to-server delivery and is blocked by most ISPs for outbound use.

What is the difference between port 587 and port 465?

Port 587 uses STARTTLS - the connection starts unencrypted and upgrades to TLS. Port 465 uses implicit TLS - the connection is encrypted from the start. Both are secure. Port 587 is the current standard; port 465 is older but still widely supported.

SMTP Port Checker - Test Email Port Connectivity

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Live DNS - no cached APIs

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Privacy first diagnostics

Prevents blacklisting - not causes it

Test whether SMTP ports 25, 587, and 465 are open and reachable on a mail server. Useful for diagnosing why email is not sending or receiving when the DNS records look correct.

SMTP ports explained

Port	Name	Use	Encryption
25	SMTP	Server-to-server email delivery (MX delivery). Also used for outbound relay in some setups.	Plain / STARTTLS
587	Submission	The recommended port for sending email from apps, mail clients, and services. Requires authentication.	STARTTLS (required)
465	SMTPS	Implicit TLS submission. Used by some providers as an alternative to 587. Older standard, but still widely supported.	TLS (implicit)

When SMTP ports matter for deliverability

Port 25 blocked outbound - Most ISPs and shared hosting providers block outbound port 25 to prevent spam. If your application tries to send directly on port 25, it will fail silently. Use an SMTP relay service (SendGrid, Mailgun, Postmark) on port 587 instead.
Port 587 not open on your mail server - If you run your own mail server and port 587 is closed, your mail clients and applications cannot authenticate and submit email. Check your firewall.
Port 25 closed on your inbound server - If your mail server does not accept connections on port 25, other mail servers cannot deliver email to you. Check your MX records point to the right server and that port 25 is open inbound.

Which port should I use to send email?

Use port 587 with STARTTLS for all outbound submission from applications, email clients, and sending services. This is the current standard recommended by RFC 6409. Port 465 (implicit TLS) is also acceptable and supported by most modern providers. Port 25 is reserved for server-to-server delivery and is blocked by most ISPs and hosting providers for outbound use.

FAQ

Port 25 is showing as blocked - is that a problem?

Not for sending. Port 25 being blocked outbound is standard practice on shared hosting and residential ISPs to prevent spam. Use port 587 or 465 via an SMTP relay service like SendGrid, Mailgun, or Postmark. For receiving email (inbound delivery), port 25 must be open on your mail server - if it is blocked inbound, no email will be delivered to your domain.

My DNS records look correct but email is not being delivered - what to check?

After confirming MX records are correct, check that port 25 is actually reachable on the mail server in your MX record. A firewall blocking inbound port 25 is a common cause of non-delivery that does not show up in DNS checks.

What does a timeout mean vs closed?

A closed port means the server actively refused the connection. A timeout usually means a firewall is dropping the connection silently rather than rejecting it. Both mean the port is not usable, but the cause is different: closed means the service is not running; timeout usually means a firewall rule is blocking it.