{"id":154809,"date":"2021-05-13T15:59:00","date_gmt":"2021-05-13T13:59:00","guid":{"rendered":"https:\/\/spotler.com\/blog\/what-is-smtp"},"modified":"2026-01-16T11:52:52","modified_gmt":"2026-01-16T10:52:52","slug":"what-is-smtp","status":"publish","type":"blog","link":"https:\/\/spotler.com\/en-de\/blog\/what-is-smtp","title":{"rendered":"What is the Simple Mail Transfer Protocol (SMTP)?"},"content":{"rendered":"

To deliver emails from A to B, virtually all emails worldwide use SMTP. This Simple Mail Transfer Protocol ensures that all mail servers speak the same ‘language’ to understand each other when transferring messages. In this article, we’ll explain how this works.<\/strong><\/p>\n

What is SMTP?<\/h2>\n

The\u00a0Simple Mail Transfer Protocol<\/strong>\u00a0is a relatively simple – hence the name – text-based protocol for sending emails. It specifies the message’s sender, recipient(s), sending details, and content. This data lets the receiving email client know who to send the message to and what to display to them. Although SMTP is not the only way to send emails, it is the\u00a0de facto<\/em>\u00a0standard for email communication.<\/p>\n

One language for all email traffic<\/h2>\n

The fact that SMTP is the standard for email delivery ensures that almost all mail servers understand each other. As a result, you can assume that an email you send to a German customer will arrive in the same fashion as an email you send to someone in the UK. <\/p>\n

However, SMTP is a protocol that assumes the recipient is always online. Since most customers aren’t online 24\/7, SMTP needs some assistance. POP3 and IMAP ensure the recipient can retrieve the email (Gmail, Outlook, etc.). With SMTP, you upload<\/strong> your email to ‘the cloud’, where POP3 or IMAP downloads<\/strong> that email.<\/p>\n

\"\"<\/figure>\n

The technical operation of SMTP<\/h2>\n

SMTP is a protocol that operates in a\u00a0client-server model<\/strong>. The client\u00a0C<\/em>\u00a0takes the initiative to communicate with the server\u00a0S<\/em>, which is always available and reactive. The client is the sending party, and the server is the receiving party. The communication between the two looks somewhat like this:<\/p>\n

Example of the SMTP client-server communication<\/h3>\n

Suppose you want to deliver an email to a client who uses Gmail. The client will connect to smtp.gmail.com. After establishing the connection via telnet (telnet smtp.gmail.com 587), the server (S) opens with a greeting:<\/p>\n

S <<\/em> 220 smtp.gmail.com ESMTP
C ><\/em> HELO flowmailer.net
S <<\/em> 250 Hello<\/code><\/pre>\n
Next, the client introduces itself by telling who the sender is. The server checks if the sender is OK.<\/p>\n
C ><\/em> MAIL FROM: you@yourcompany.com
S <<\/em> 250 Ok<\/code><\/pre>\n
Now that the server has approved the sender’s address, the client tells who the recipient is. Again, the server approves this.<\/p>\n
C ><\/em> RCPT TO: yourcustomer@gmail.com
S <<\/em> 250 Ok<\/code><\/pre>\n
The email content<\/strong><\/h4>\n
Now that the server is familiar with the client, the sender, and the recipient, the client can start sharing data. This data consists of the subject line, the sender, the recipient, and the message’s content. It looks something like this:<\/p>\n
C ><\/em> DATA
S <<\/em> 354 Enter message, finish with '.' on a line by itself
C ><\/em> SUBJECT: Your order is confirmed!
C ><\/em> From: you@yourcompany.com
C ><\/em> To: yourcustomer@gmail.com
C ><\/em>
C ><\/em> Contents of e-mail
C ><\/em> .<\/code><\/pre>\n
Closing and terminating SMTP connection<\/h3>\n
After transferring the data, the client’s job is done. After receiving a 250 OK response from the server, the connection terminates, and the email queues for delivery.<\/p>\n
The analogy of the mailman<\/h3>\n
You’ll notice two senders and recipients in the example above (‘MAIL FROM’ & ‘RCPT TO,’ versus ‘From’ & ‘To’). This situation is similar to a regular envelope, where a mailman only sees the envelope (‘MAIL FROM’ & ‘RCPT TO’), and the addressee reads the letter (‘From’ & ‘To’). The mailman doesn’t check the contents of the letter. That poses a problem. What if the envelope has a different sender on it than the letter itself?<\/p>\n
Limitations of SMTP<\/h2>\n
SMTP is somewhat insecure and prone to spam. Since the old-fashioned protocol doesn’t check the validity of the email per se, it is possible to fake your way to the customer’s inbox. You can use a different name in the MAIL FROM and the ‘From’ header, otherwise known as spoofing. Over the years, however, security measures were developed to protect both the MAIL FROM and the ‘From’-header.<\/p>\n
SPF & DKIM<\/h3>\n
To protect their MAIL FROM address, domain owners can use the Sender Policy Framework (SPF). With SPF in place, the servers can now check if the MAIL FROM is valid. It checks if the sending IP address is authorised to send emails on behalf of the proposed sender.<\/p>\n
DKIM, on the other hand, protects the From header. Using a cryptographic signature, DKIM ensures the sender address and email content are valid. It ensures that the recipient sees what they’re supposed to.<\/p>\n
DMARC<\/h3>\n
With SPF and DKIM in place, you can ensure that your domain is protected. With DMARC, you prevent domain spoofing for good. DMARC specifies what should happen to emails that fail the SPF and DKIM checks.<\/p>\n
ESMTP<\/h3>\n
All these measures would not have been possible if SMTP hadn’t extended its functionality. To have more functionality than ‘just send and receive emails,’ ESMTP (E for Extended) was created in 1995. This variant of SMTP offers the possibility of email authentication and performance improvement.<\/p>\n
\"\"<\/figure>\n
Summarised<\/h2>\n
SMTP is one of the oldest, but still the most common, email delivery methods. Though the protocol has made massive improvements over the years, it’s not perfect from the start. You must authenticate your domain, improve your IP reputation, and ensure the path to the inbox is as fast and secure as possible.<\/p>\n","protected":false},"excerpt":{"rendered":"
SMTP is used in sending and receiving email. But what is it exactly, how does it works and what are the limitations? We give you an answer!<\/p>\n","protected":false},"author":46,"featured_media":77625,"template":"","cat_industry":[1943,1459],"cat_topic":[1635],"class_list":["post-154809","blog","type-blog","status-publish","has-post-thumbnail","hentry","cat_industry-b2b-en-de","cat_industry-b2c-en-de","cat_topic-deliverability-en-de"],"acf":[],"_links":{"self":[{"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/blog\/154809","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/blog"}],"about":[{"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/types\/blog"}],"author":[{"embeddable":true,"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/users\/46"}],"version-history":[{"count":3,"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/blog\/154809\/revisions"}],"predecessor-version":[{"id":207362,"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/blog\/154809\/revisions\/207362"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/media\/77625"}],"wp:attachment":[{"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/media?parent=154809"}],"wp:term":[{"taxonomy":"cat_industry","embeddable":true,"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/cat_industry?post=154809"},{"taxonomy":"cat_topic","embeddable":true,"href":"https:\/\/spotler.com\/en-de\/wp-json\/wp\/v2\/cat_topic?post=154809"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}