What is an SMTP?
Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending and receiving e-mail on a Transmission Control Protocol (TCP)/IP network. SMTP provides the ability to send and receive email messages via Standard SMTP Port.
SMTP is an application-layer protocol that facilitates the transmission and delivery of email over the Internet. SMTP is designed and sustained by the Internet Engineering Task Force (IETF).
SMTP is also known as RFC 821 and RFC 2821.
SMTP is usually integrated with an email client application and is formed of four key components:
- Mail user agent (MUA) – local user or client-end service
- Mail submission agent (MSA)
- Mail transfer agent (MTA)
- Mail delivery agent (MDA)
SMTP operates by opening a session between the user and server, whereas MTA and MDA give domain searching and local delivery services.
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What is an SMTP Port?
Let’s start with what is a port.
The simplest approach of explaining this is to think of your router/modem as the door to the internet from your home or office. Now, imagine rather of one door you have 65,535 doors built into that router/modem and each door allows one function. By examining what doors are open and shut, you check what kind of traffic can come in and leave your home/office. You can have that door point towards a particular place in the house only, so you examine not only who comes in and leaves, but where they come in to or where they can go.
Out of 65,535 doors, door number 25 i.e Port 25 allows mail to flow in and out and is called (Simple Mail Transport Protocol – SMTP Port).
An SMTP port is one that is intended to be used for SMTP connections. Today, the most familiar SMTP ports are 25, 465, 587, or 2525.
What is the Standard SMTP Port?
The standard port used for SMTP connections is SMTP Port 25.
Mail250 too offers alternative SMTP ports – 2525, 587. For secure SMTP over SSL you can use port 465.
How are these ports different?
SMTP Port 25:
Port 25 is the oldest of the four. It was the port number assigned to SMTP when the protocol was first introduced in the now obsolete RFC 821 back in 1982, about 37 years ago.
This SMTP port remains to be used primarily for SMTP relaying. SMTP relaying is the transmittal of email from the email server to the email server.
Several ISPs, however, block port 25, that’s why we suggest using one of the alternative SMTP ports.
SMTP Port 465:
This SMTP port 465 was originally added when users started looking for ways to secure email messages.
IANA has reassigned a different service to this SMTP port, and it should no longer be utilized for SMTP communications.
However, because it was once approved by IANA as valid, there may be legacy systems that are only able to use this connection method. Typically, you will use this SMTP port only if your application requires it. A quick Google search, and you’ll notice many consumer ISP reports that recommend SMTP port 465 as the advised setup.
SMTP Port 587:
Port 587 is the one recommended for mail submission. When a mail client is submitting an email to be routed by a proper mail server, it should always use this port 587.
In fact, this is specified which states that “Port 587 is reserved for email message submission…”.
You should consider using this port as default, except you’re explicitly blocked by your hosting provider. This port, copulated with TLS encryption, will assure that email is submitted securely and following the guidelines set out by the IETF.
SMTP Port 2525:
SMTP Port 2525 is not an official/authenticated port, also it is not endorsed by the IETF nor IANA. Instead, Mail250 provides it as an alternate port, which mirrors port 587, in the event the above ports are blocked.
Because SMTP Port 2525 is a non-traditional special port number, it is typically provided on consumer ISPs and Cloud Hosting providers, like Google Compute Engine. If you’ve tried the above SMTP ports, but encounter connectivity problems, try port 2525.
This SMTP port 2525 also supports TLS encryption.
Sources:
- IANA Service Name and Transport Protocol Port Number Registry
- “Revoking the smtps TCP port” – Email from Internet Mail Consortium director Paul Hoffman, 12 Nov 1998.
- RFC 6409 – Message Submission for Mail
- RFC 5321 – Simple Mail Transfer Protocol
- RFC 3207 – SMTP Service Extension for Secure SMTP over Transport Layer Security
- RFC 4607 – Source-Specific Multicast for IP
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