What is port in SMTP?

A port in SMTP refers to the networking endpoint used for communication between email servers and clients. Ports allow multiple logical connections to be made simultaneously with a single physical network interface. Standard SMTP utilizes port 25 while variants use additional ports for specific capabilities.

What is port in SMTP?

Overview of Ports in Networking

In computer networking, a port serves the following functions:

  • Provides logical communication endpoint for services and network connections.
  • Allows many simultaneous sessions over a single network interface by multiplexing.
  • Used along with IP address for identifying service on host.
  • Enables distinguishing traffic meant for different services like HTTP, SMTP etc. on same server.
  • Common ports standardized but any unused ports can be used if required.

So in essence, ports enable running multiple networking services and connections in parallel on hosts identifiable by unique port numbers.

Standard SMTP Ports

Some common SMTP-related ports are:

  • Port 25 – Default port for basic SMTP traffic. Unencrypted but still widely used.
  • Port 465 – Implicit TLS-encrypted SMTPS connections. Provides additional security.
  • Port 587 – Designated for SMTP mail submission from clients to servers.
  • Port 2525 – Alternative submission port sometimes used instead of 587.
  • Port 24 – SMTP connections over Private Networks e.g. between mail gateways.

So while 25 is standard, alternatives cater for encrypted and submission needs.

Working of SMTP Port 25

SMTP port 25 operates as follows:

  • Servers listen for inbound SMTP connections from other servers and clients on TCP port 25 by default.
  • Clients initiate outbound SMTP sessions by connecting to remote server port 25 for sending emails.
  • Server-server, server-client conversations occur over established connections.
  • SMTP commands and responses exchanged between sender and recipient systems over port 25.
  • Email accepted from client gets delivered to destination using routing based on MX records priority.

So port 25 provides common ground for SMTP sessions enabling transmission of mail between systems.

Purpose of Standard SMTP Port 25

There are some key reasons for standardized use of port 25 for core SMTP:

  • Provides consistent endpoint for origination and delivery of SMTP conversations.
  • Enables mail clients and servers to locate SMTP daemons remotely based on fixed conventions.
  • Limits need for specifying port details during handshakes. 25 is assumed if unspecified.
  • Simplifies network security rules for enabling SMTP since most firewalls recognize port 25.
  • Provides single target for interoperability irrespective of internal system details and platform.
  • Avoid port conflicts between multiple services by dedicating 25 for SMTP channel.

So for ubiquity, port 25 emerged as the ubiquitous SMTP port for guaranteed client-server mail exchange everywhere.

Working of SMTP Submission Port 587

Port 587 serves specific purpose in SMTP architecture:

  • Intended for email submission from Mail User Agents to Mail Submission Agent.
  • Separates submission of outgoing mails from incoming delivery over port 25.
  • Allows impose extra validation policies on email contributed by users only.
  • Helps block spammers by restricting unauthorized connections unlike default port 25 which is open.
  • Port 465 can also sometimes be used for secure SMTP submissions instead of port 587.

So port 587 selectively handles user-initiated SMTP sessions for better security.

Using Standard vs Submission Ports

There are some guidelines on using standard versus submission ports:

  • Port 25 – For bi-directional communication between MTAs. Best suited for server-to-server SMTP delivery conversations.
  • Port 587 with authentication – For user’s Mail User Agent applications to submit mails which get handed over to port 25 for delivery through routing. Provides receiver controls.
  • Port 25 outgoing, Port 587/2525 incoming – Enables granular control over inbound vs outbound traffic plus user submissions.

So standard versus submission ports usage depends on source-destination context and needs around isolation and security controls.

Working of SMTPS Implicit TLS Encryption Port

SMTPS works on principles of implicit TLS encryption:

  • Uses non-standard port 465 by default unlike plain SMTP.
  • Encrypts SMTP session right from the initial handshake during connection establishment.
  • Upgrades entire conversation to TLS without needing separate STARTTLS command.
  • Mandates both parties support TLS by virtue of connecting on port 465 compared to opportunistic encryption in STARTTLS.
  • Offers enhanced security and mitigates risks of transmitting credentials or messages in plaintext.

So SMTPS port 465 connections get end-to-end protection automatically.

Standard SMTP vs SMTPS Implicit TLS Port

Standard SMTP Implicit TLS SMTP
Uses traditional port 25. Utilizes port 465 by default.
Unencrypted communication. Encrypts full session mandatorily.
No identity verification. Server identity validated by certificate.
Opportunistic TLS upgrade via STARTTLS. Secured right from initial handshake itself.
Prone to interception of usernames/passwords and messages. Resilient to sniffing, spoofing, tampering risks.
Compatible with older systems lacking TLS. Requires TLS support on both client and server side.

So SMTPS connections build transport layer security into the core protocol unlike opportunistic protection through STARTTLS.

Key Considerations for SMTP Port Usage

Some key aspects to evaluate when utilizing SMTP ports:

  • Encrypt connections by default using implicit TLS ports like 465 or TLS-secured submission ports. Avoid plain text where possible.
  • Authenticate users submitting mail by enforcing validation policies on submission ports.
  • Close down unneeded open relays by tightening firewall rules.
  • Follow principle of least privilege by exposing only ports necessarily required for core functions.
  • Implement TLS on standard ports opportunistically if encrypting entire session is not possible.
  • Monitor security infrastructure like IDS/IPS systems for anomalous port activities.
  • Set up SMTP recipient/sender allow-lists on mail gateway restricting IP access to harden environment.

So fine-tuned controls and oversight helps securely expose only necessary SMTP ports in any deployment.

Key Takeaways on SMTP Ports

  • Port 25 is the standard for core SMTP traffic. Other alternatives like 587, 465 cater to specific needs.
  • Port587 separates user submission from general SMTP delivery for better security.
  • Port 465 mandates immediate TLS encryption unlike opportunistic upgrades in port 25.
  • SMTP port usage choices depend on source-destination, encryption and validation needs.
  • Lock down exposure, implement least privilege access, enable encryption opportunistically if not by default on ports.


While port 25 remains the core SMTP endpoint, additional alternatives like 465 and 587 enable enhanced security through mandatory encryption and tighter policy controls respectively. For internet-facing deployments, implicit TLS ports that warrant transport layer protection right from initial handshake as well as providing receiver-based controls are vital for robust mail delivery. Inside organizations, inter-server and client-server port usage need thoughtful isolation tailored to infrastructure needs for stability. With email’s enduring relevance, care around exposing only necessary ports, hardening configurations and monitoring activity remains key to long-term SMTP reliability across expanding organizational boundaries.


  1. What happens if SMTP port 25 is blocked?
    If port 25 is blocked, standard SMTP delivery fails but alternatives like port 587 for submission are still usable for sending outgoing mails after configuration adjustments.
  2. Can SMTP work on ports other than 25?
    Yes, while 25 is standard, SMTP variants operate on other ports too like 587 for submissions, 465 for SMTPS etc. but need appropriate server and client support.
  3. Which is better SMTP or SMTPS port?
    SMTPS is better as it encrypts the connection end-to-end by default unlike SMTP which upgrades to TLS only optionally. But both have valid uses cases.
  4. Is port 25 secure if I use username and password?
    No, using credentials over unencrypted port 25 only safeguards against unauthorized usage but not snooping. TLS encryption is still needed to protect username, password and data in transit.
  5. What are the most common ports for email?
    25 for SMTP, 465/587 for secure SMTP submission, 110/995 for POP3, 143/993 for IMAP are most common ports associated with email services.
  6. Can I run both TLS and non-TLS SMTP together?
    Yes, you can run TLS and non-TLS SMTP concurrently using separate ports like 25 for non-TLS and 465/587 for implicit TLS configurations on same or different servers.
  7. What ports do email clients use?
    Email clients connect to SMTP ports like 25, 587 for sending mail and ports 110, 143 for fetching mail from servers using IMAP/POP3. 443 also commonly used for webmail access.
  8. How do I know if SMTP server is listening on a port?
    You can check if an SMTP server is listening on a specific TCP port by using the telnet command like ‘telnet mail.domain.com 25’. Connection response confirms operation.
  9. Can I run SMTP on non-standard high ports?
    You can technically run SMTP on any unused non-standard ports but mail clients may fail to connect unless specifically configured since they assume standard ports by default.
  10. Is port 25 blocked on residential ISP networks?
    Often yes, many residential ISPs block outbound port 25 to discourage home servers. Port 587 used as alternative submission for home users while business connections offer unblocked port 25.
  11. Can closing port 25 cause problems?
    Yes, incorrectly closing port 25 will break core SMTP functionality leading to failures in sending and receiving emails. Leave port 25 open on mail servers.
  12. Is it okay to run SMTP on public IP addresses?
    No, exposing port 25 on public IP without firewall protection risks abuse by spammers. Use IP allow-lists, block unused ports, enable TLS etc. to secure public servers.
  13. How does port 25 work with a firewall?
    Firewalls need to allow outbound TCP 25 connections for email delivery from internal mail servers to external domains. But restrict inbound port 25 externally to avoid abuse.
  14. Do email clients need to use port 25?
    Email clients need to connect to submission ports like 587 for outbound mail, not port 25 which is meant for inter-server communication. But TCP port 25 connections are needed for receiving mail.
  15. What are the security issues with SMTP port 25?
    Lack of default encryption on port 25 makes interception of usernames, passwords and mail data possible. It also allows spoofing without validating identities.
  16. Can I run a mail server on a different port than 25?
    You can technically run mail server on other ports but port 25 is the standard that clients expect. Use other ports along with port 25 instead like 465 for SMTPS or 587 for submissions.
  17. Is port 25 required for sending emails?
    Port 25 is not absolutely mandatory for sending mail but configured submission port like 587 is required. But port 25 is widely used for server-server routing of messages internally.
  18. Can I block port 25 on my firewall?
    You can block external port 25 through firewall but any internal mail servers hosted in LAN still need to be able to connect outbound via port 25 for sending emails outside otherwise delivery will fail.
  19. How do spammers abuse port 25?
    Spammers exploit open port 25 relays that do not validate recipients to route high email volumes anonymously making it vital to safeguard port 25 through strict allow-lists.
  20. Why SMTP submission and port 25 needed?
    Port 587 submission provides sender validation and specialized handling while port 25 offers general bi-directional communication between MTAs for standard message routing and delivery.

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