What port is SMTP?

The Simple Mail Transfer Protocol (SMTP) is a standard protocol used for sending emails across the internet. SMTP utilizes TCP port 25 for transporting email messages between mail servers.

What port is SMTP?

Why SMTP Uses Port 25

Port 25 was assigned as the default TCP port for SMTP in RFC 675 back in 1974. At the time, port 25 was simply chosen as an available standardized port for the newly created SMTP protocol.

Other common protocols you may be familiar with also use standardized ports defined in various RFCs:

    HTTP uses port 80

    HTTPS uses port 443

    FTP uses port 21

These standardized ports allow different computers to know how to communicate with each other across networks. For SMTP, using the standard well-known port 25 allows any email client or server to easily connect to any other SMTP mail server for sending emails.

How SMTP Port 25 Works

Understanding the basics of how SMTP works explains why port 25 is so integral to the email sending process:

    A user sends an email from their email client (Outlook, Gmail, etc).

    The outgoing mail server attached to that email client initiates an SMTP connection over port 25 to the recipient’s mail server.

    The sender’s mail server transports the email through this SMTP connection to the recipient’s receiving mail server.

    The recipient’s mail server delivers the email to their inbox.

Without an open SMTP port 25 on both servers, this email sending conversation would fail.

Key Takeaway: SMTP servers must have TCP port 25 open and listening for connections to successfully send outgoing emails. Client email services (Outlook, Gmail, etc) connect to these outgoing SMTP mail servers for transporting emails to recipients.

SMTP Port 25 Alternatives

In some cases, SMTP mail servers use alternative ports for outgoing mail transport besides the standard port 25:

    Encrypted SMTP – Servers that support encrypted protocols like SMTPS (SMTP over SSL) typically use non-standard SMTP ports like 465 or 587 instead of port 25. However encrypted SMTP is still relatively uncommon on most mail servers internet-wide.

    ISP Blocking – Some ISPs completely block access to port 25 to reduce spam and malicious emails originating from their networks. So servers at these ISPs will be forced to use a non-standard alternate port if configured as outgoing SMTP mail servers.

    Anti-Spam Restrictions – Services like Gmail may restrict using the standard SMTP port 25 when connecting out to other mail servers as an additional anti-spam measure.

However, these situations are more exceptions to the rule. The vast majority of SMTP sessions still occur over the expected standardized port 25 for transporting emails across the internet.

Network Communication Basics

Before going deeper into how port 25 powers email sending, it helps to understand some basic principles of computer networking:

    Computers communicate by establishing connections between each other over a network.

    These connections allow sending data back and forth by packaging it into packets.

    To allow many simultaneous connections over the same network, there needs to be a way to direct this packet traffic.

    So connections rely on addressing schemes using ports and IP addresses:

        IP Addresses – Identify each connected device on a network with a unique numbered address.

        Ports – Let a single device have multiple connection points by assigning a specific port number to each communication session connected to that device.

With hundreds or thousands of devices on a typical network, ports + IP addresses allow properly routing message traffic coming to and from all these computers simultaneously while keeping separate sessions compartmentalized.

Putting this together with an SMTP example:

    The outgoing mail server at 123.123.123.123 establishes an SMTP connection over port 25 to the incoming mail server at 456.456.456.456

    This allows packaging an outgoing email into packets addressed specifically to port 25 on the target 456.456.456.456 server.

    The receiving server listens on port 25, accepts this SMTP connection, and receives the packets containing the email message for delivery.

So the standard SMTP port 25 allows this necessary communication channel for transporting email messages.

How an SMTP Connection Works

Digging deeper into the technical details, here is an overview of what a full SMTP handshake looks like using port 25:

    Email client contacts its configured outgoing SMTP mail server to initiate sending an email.

    Local SMTP server establishes a TCP connection on port 25 to the recipient’s incoming mail server.

    SMTP handshake initiates with greeting messages:

        220 mail.recipientdomain.com SMTP Ready

        EHLO outgoing.senderdomain.com

        250 Accepted

    Email sender identifies themselves:

        MAIL FROM: [email protected]

        250 [email protected] Accepted

    Email recipient is specified:

        RCPT TO: [email protected]

        250 [email protected] Accepted

    Sender transmits email data:

        DATA

        354 Enter Message, Ending With “.” on a line by itself

        Email headers, body contents, attachments sent

        Terminating . sent

        250 Message accepted for delivery

    Session exited:

        QUIT

        221 mail.recipientdomain.com closing connection

        Connection closed

As you can see, this inbound delivery of an email relies entirely on successfully connecting to remote port 25 to establish an active SMTP session. Without port 25 communication in both directions, the email would never leave the originating server to reach its destination.

Practical Applications

In summary, understanding that SMTP utilizes port 25 as its fundamental communication mechanism helps explain several common issues that can interrupt or block emails:

    Port 25 blocked – If the local network blocks outbound port 25, the mail server cannot transport outgoing messages. Servers in this situation should be reconfigured to use a non-standard SMTP port permitted through the firewall.

    Destination port 25 closed – If the remote receiving server has not opened port 25, or actively closed it, no SMTP connection can complete so messages will fail to send. The recipient server needs to start listening on port 25 for inbound delivery to function.

    Hostname misconfiguration – SMTP hostnames must resolve correctly to connect out from port 25 to the proper receiving mail server on the other end. Otherwise packages end up lost with no route to the destination IP address.

    Connection timeouts – Packet loss, network problems, congestion issues could disrupt an active SMTP session if the port 25 communication gets interrupted. Engineers then need to troubleshoot the network environment.

So in essence, port 25 forms the backbone communication channel enabling the SMTP email sending process from end to end. Failing to establish this SMTP connectivity due to port 25 issues is one of the most common reasons for email delivery problems Internet-wide.

Understanding port 25 provides practical insight for troubleshooting and preventing email failures.

Conclusion

In summary, SMTP uses port 25 by standard specification since the earliest days of email. This well-known port allows easy intercommunication between SMTP mail servers across the Internet for delivering emails from senders to recipients. Learning the details of how SMTP messaging relies on port 25 connectivity helps explain many common email issues that can interrupt successful message transmission. Troubleshooting port 25 should be the first step for engineers, system administrators, or IT specialists dealing with email delivery problems.

Frequently Asked Questions

Q: What is port 25 used for?
A: Port 25 is used for Simple Mail Transfer Protocol (SMTP) email delivery between mail servers on the Internet. This port must be open for sending outgoing emails.

Q: Why was port 25 chosen for SMTP?
A: Port 25 was arbitrarily chosen as a standardized port when SMTP was specified in RFC 675 in 1974. Being defined in the original protocol RFC is why 25 remains the standard SMTP port after nearly 50 years.

Q: What network protocol uses port 25?
A: The SMTP (Simple Mail Transfer Protocol) specifically uses and relies on port 25 for efficiently routing email messages between sending and receiving mail servers across the Internet.

Q: Is port 25 TCP or UDP?
A: Port 25 uses TCP as the transport layer protocol. SMTP requires maintaining stable, connected sessions between mail servers which TCP enables unlike connectionless UDP.

Q: Can SMTP use other ports?
A: SMTP alternate ports like 465 (SMTPS) or 587 (submission) exist but port 25 remains the primary standard for normal SMTP mail transport even today. Some networks block 25 forcing exceptions.

Q: Is port 25 required for sending emails?
A: In most cases yes, virtually all mail servers need port 25 functionality for public Internet email delivery to external domains. Internal mail within a private network can sometimes rely on alternate ports instead.

Q: What happens when port 25 is blocked?
A: Blocking port 25 prevents outbound SMTP connections for sending emails. Messages may queue but will eventually bounce without the ability to establish port 25 transport to external receiving mail servers.

Q: Why would my ISP block port 25?
A: ISPs often block port 25 to reduce the amount of spam and malicious emails sent from their networks. But this also impacts legitimate delivery. Better anti-spam controls through port 25 are preferred.

Q: How do I know if port 25 is blocked by my ISP?
A: Try telnetting from the command line to various external mail servers on port 25 from your network to see if connections succeed. If the port 25 session immediately resets without an SMTP banner response, outbound port 25 is likely blocked.

Q: Can I use Gmail if port 25 is blocked?
A: Yes, Gmail transmits emails by connecting outbound via SMTP itself. So you can still send receive emails using Gmail, Hotmail, or other cloud email providers even with port 25 blocked on your local network.

Q: What is a normal SMTP port 25 response?
A: A 220 response code at the start indicates an open and listening SMTP server. 220 mail.serverdomain.com ESMTP Postfix is typical for a healthy port 25 connection.

Q: Why does my email fail when port 25 is open?
A: Beyond port 25 connectivity, domain name resolution, reverse DNS records, firewall security rules, TLS configuration, relay restrictions, authentication problems, and more can also disrupt SMTP sessions.

Q: What does it mean to throttle SMTP port 25?
A: Throttling port 25 applies bandwidth limits to overall SMTP traffic volumes to restrict the number of emails capable of being sent per hour. This intentional slow down helps limit spam and email attacks.

Q: What port should I specify for my SMTP server?
A: You should configure your outward facing SMTP server to listen on the standard port 25 for ordinary non-encrypted email delivery to properly receive emails from external domains.

Q: What gets logged in SMTP port 25 communications?
A: By default SMTP sessions log basic send and receive envelopes, mail headers, client IPs and hostname, and protocol transcripts useful for troubleshooting or auditing port 25 transactions .

Q: Can I change the default SMTP port 25?
A: SMTP port 25 cannot itself be changed since it is hard-coded protocol specification. But alternative ports used instead of 25 like 465 or 587 help sidestep port 25 limitations when standard SMTP is problematic.

Q: Is port 25 secure?
A: No. Port 25 SMTP transmits email messages in plain text allowing potential eavesdropping. Use SMTPS encryption or VPNs to secure port 25 transport instead of standard SMTP.

 

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