---
title: What is Port 587 Used For? SMTP Submission Port Explained | SendHQ
description: Technical explanation of SMTP port 587, its role in email submission, the use of STARTTLS, and how it differs from ports 25 and 465.
canonical: https://sendhq.cc/terms/what-is-port-587-used-for
last-updated: 2026-08-26
---
# What is Port 587 Used For? SMTP Submission Port Explained

Port 587 is used for SMTP message submission, allowing email clients to send outgoing mail to a mail server. It is the standard port for client to server communication and typically requires authentication and the use of STARTTLS to encrypt the connection.

## The Role of Port 587

Port 587 is designated as the submission port for SMTP. Unlike port 25, which is primarily used for server to server relay, port 587 is intended for end users to submit messages to their mail server. This separation helps prevent spam by ensuring that clients authenticate before their messages are accepted for delivery to the final destination.

## STARTTLS and Encryption

Connections on port 587 typically begin as plain text and are upgraded to a secure TLS connection using the STARTTLS command. This mechanism ensures that credentials and message content are encrypted during transit. This differs from implicit TLS, where the connection is encrypted from the first byte, which is the behavior associated with port 465.

## Comparison with Port 25 and 465

Port 25 is the legacy SMTP port used for relaying mail between Mail Transfer Agents. Many ISPs block port 25 for residential and cloud IP ranges to stop botnets from sending spam. Port 465 was originally intended for SMTPS but is now considered deprecated in favor of port 587 with STARTTLS, though many providers still support it for implicit SSL/TLS.

## Implementation and Troubleshooting

When configuring an email client or application, select port 587 and enable TLS or STARTTLS. If the connection fails, verify that your firewall allows outbound traffic on this port. Developers can use SendHQ free tools (https://sendhq.cc/tools) to verify their DNS and authentication records to ensure mail submitted via port 587 is not flagged as spam.

## Authentication Requirements

Most servers listening on port 587 require SMTP AUTH. This means the client must provide a valid username and password before the server will accept the message for relay. This requirement prevents open relays, which are servers that allow anyone to send mail, making them primary targets for spammers.

## Questions teams ask

**Is port 587 secure?**

Yes, when used with STARTTLS, port 587 encrypts the communication between the client and the server, protecting sensitive data from interception.

**Should I use port 587 or 465?**

Port 587 is the current IANA standard for mail submission. Port 465 is used for implicit TLS and is less common in modern standard configurations.

**Why is port 25 blocked but 587 open?**

ISPs block port 25 to prevent unauthorized mail servers and spam bots from sending mail directly from a user's network to the internet.

## Primary sources

- [Service Name and Transport Protocol Port Number Registry](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml) — IANA
- [RFC 3207: SMTP Service Extension for Secure SMTP over Transport Layer Security](https://www.rfc-editor.org/rfc/rfc3207) — RFC Editor
- [RFC 8314: Cleartext Considerations for Post-SMTP Email Protocols](https://www.rfc-editor.org/rfc/rfc8314) — RFC Editor

## Continue learning

[smtp port](https://sendhq.cc/guides/smtp-port.md) [mail protocol smtp](https://sendhq.cc/guides/mail-protocol-smtp.md) [smtp relay service](https://sendhq.cc/guides/smtp-relay-service.md) [free smtp service](https://sendhq.cc/guides/free-smtp-service.md)