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When you send an email, it doesn’t go directly from the your device to the recipient. Instead, it travels between many different computers on its way. Its first port of call is an SMTP server. In order to communicate with this server, you must use the SMTP protocol.


SMTP stands for “Simple Mail Transfer Protocol” which is the standard technology we use when sending email. Although other technologies have been developed, this simple, text-based protocol still remains the norm.


Here’s an example of a communication between a sender and a server when sending an email.

First, the sender (the client) makes a TCP connection on the appropriate host port. Most of the time we use port 25. Our service is also available on 8025, 587, 80, 465, 8465 and 443.

We can achieve this on most devices by simply entering the following command at a command prompt. Here, we’ll use smtp2go.com as an example server.

  telnet smtp2go.com 25


Check out the actual communication below (S: stands for server and C: stands for client)

Sample SMTP Communication


When sending emails, we use two types of ‘greetings’, HELO and EHLO. Most clients (e.g. Outlook, Eudora Mail) will use EHLO.

EHLO invokes an Extended SMTP (ESMTP). We use it to enable more options and to see which extensions are supported by the server. However, if the server fails to respond to the EHLO command, the client can use HELO. HELO will always work.

One of the benefits of using the extended form is that you can determine in advance whether a server will be able to send an email of a certain size. Some outdated clients and servers reject large messages, but but only after the server attempts for some time to force the email through.

Extended SMTP

If  you see a number printed next to the SIZE keyword, it indicates the maximum allowed size of a message. If no number is printed, additional interaction can be initiated by the user to test if they can send their message.


Formally SMTP is defined in RFC 821 as amended by RFC 1123. Modern clients use the protocol known as ESMTP and is defined in RFC 2821.


The main problem many people have with sending emails is that most sending servers require you to always connect to the internet through the ISP that provides the server. For mobile users this is often impossible. SMTP2GO allows you to send from any city or country, anywhere in the world, no matter where you are or what device you’re using.

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