What is SMTP? How does it work, and does it matter? incl. tools

Email marketing is a powerful channel. Its average ROI is 4,200%. But you must create and send emails the right way to get close to this average. This means you have to care for how they look and how you set them up technically.

The technical side includes servers and email protocols, like SMTP. So what is SMTP, and why do you need it? Let’s talk about everything you need to know about it.

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is a protocol or a set of rules that email clients use to exchange information. It sends your emails to the recipients. So after you hit ‘Send,’ the protocol pushes your message to the SMTP servers of your email client. Then, it goes to your recipient’s mail server. The SMTP servers receive and deliver it to the recipient’s inbox. 

SMTP is the first email protocol on an email’s journey from a sender to a recipient. It’s the only protocol for sending messages. There are also IMAP and POP3 email protocols. These are for receiving messages. We’ll discuss them in detail later.

Using an SMTP allows for authenticating your email address. A verified email address, in turn, brings benefits like:

  • higher email deliverability
  • safe bulk emails & campaigns
  • easy campaign data collection and reporting

What is an SMTP mail server?

An SMTP server is an application that uses the protocol to send your emails. It pushes the messages from your device to the SMTP servers of your recipient’s email client. It sounds like many steps for an email to go through, but the entire journey takes just a few moments.

A good way to understand an SMTP server is by looking at email messages as sending out a used clothing item you’ve just sold on eBay. Let’s say you’re selling a pair of vintage sneakers. You see someone has bought them, so you pack the contents in a box (type the mail message). You then fill in the recipient’s address (like you’d type in their email address). You print it out and bring it to a delivery service (hit ‘Send’). 

So what does an SMTP server mean in this example? Your local delivery service office is like an SMTP server. They check if the address is valid and the package matches the criteria. They follow a protocol to accept the package like the SMTP servers accept your email. Then, you go home (log out of your email client) and wait for the service to deliver your package. 

If everything’s okay with the package, the service takes care of the delivery. They send your box to the recipient’s city (recipient’s email client’s server). The city’s delivery office follows the protocol for incoming packages to accept it (similarly to IMAP and POP3). If all rules have been followed, the person receives a delivery. This is when the recipient receives incoming messages from you.

delivery of emails through the smtp server

How does SMTP work?

When you hit the ‘Send’ button on your email message, your mail user agent (MUA) connects to the SMTP server. An MUA is your email client, like Gmail or Yahoo. The connection is usually established via port 25. Check out our guide on how to send emails with Gmail SMTP.

So your mail client introduces itself to the SMTP server with a command. The MUA’s introduction, followed by the SMTP server’s reply, is called an SMTP server handshake. 

Next, the Message Transfer Agent, or Mail Transfer Agent (MTA), sees the email client you’re using to send your mail message. If it’s the same as the recipient’s one, the MTA pushes the message to the recipient’s inbound SMTP servers right away. If the mail client is different, the MTA identifies the client via the Domain Name System (DNS). It then sends it to the right server. The message is queued or sent to a backup server if SMTP servers are unavailable.

Your recipient’s email client finally gets the message. The server sends it to the person’s inbox, where they can access it.

movement of emails from the mail user agent to the smtp server

Basic SMTP Commands

An SMTP server command is text-based and is limited to 510 characters each. And what does the SMTP server do? It replies to the command with a numerical code. There are many SMTP commands the SMTP relay servers use to communicate with other servers. Here are the most used ones listed in the order they come in an SMTP relay servers conversation.


HELO is the first command that starts the conversation. It introduces the IP address or the domain of the email client to the SMTP server. If the server responds, this interaction is what we mentioned earlier as a handshake.

Client: HELO client.com
Server: 250 client.com


The EHLO command performs the same action as HELO. But only servers supporting the SMTP server service extensions recognize it. Otherwise, the server replies with an error.

Client: EHLO client.com
Server: 250 client.com


MAIL FROM starts the email transfer process — it introduces the sender’s email address. The server receives the address and checks the email client’s domain. If it’s valid and acceptable, it replies to confirm that it can move to the next step.

Mail Client: MAIL FROM [email protected]
Server: 250 OK


RCPT TO does the same as MAIL FROM but for the recipient. The command requests to confirm that the email address can receive the email. In case there are several recipients, the command repeats for each address.

Client: RCPT TO [email protected]
Server: 250 OK

rcpt to command in the smtp server


DATA identifies the message body of the email as the data that the email client needs to transfer. The SMTP server has to confirm the transmission with the 354 code. Only then the actual message transmission begins. The client marks the end of the outgoing messages with a full stop.

Client: DATA
Server: 354

Client: Date: 27 Apr 2022, 7:23:23 PM
From: [email protected]
Subject: Example Subject Line
To: [email protected]
Body Text: Hey, This is an example email body text. Bye!


The QUIT command ends the conversation between the email client and the SMTP server. To start the conversation again, the client has to send the HELO or EHLO commands.

Client: QUIT
Server: 221 OK

These are the essential SMTP server commands. But there are also additional ones like NOOP, HELP, EXPN, RSET, AUTH, ATRN, and others.

Understanding SMTP error codes

The SMTP server replies with an error code to show that an issue has occurred. The error codes point to issues like the recipient’s address doesn’t exist in the database. Or the sender email has sent spam before. Error codes in SMTP relay conversations start with a 4 or 5.

4.X.X error codes

Temporary errors start with a 4, meaning that they will be fixed soon and you can try again later. These are the temporary SMTP server error codes you can come across:

  • 421 — server is unavailable at the moment
  • 450 — the encountered mail server is unavailable at the moment
  • 451 — the server error caused a failure when sending the message
  • 452 — the server storage is full, the command has stopped
  • 455 — the server can’t reply to the command for other reasons

5.X.X error codes

Error codes for permanent failures start with a 5 and need action to be fixed. Here are the permanent issue codes to know:

  • 500 — the command contains a syntax error, the server can’t recognize it
  • 501 — the server doesn’t recognize the command because of a syntax error
  • 502 — the command is valid, but the server can’t perform it since it doesn’t support it
  • 503 — the client has sent the commands in a wrong order
  • 541 — the recipient’s address is blacklisted or has rejected the message because it violates email policy
  • 550 — the receiving server identifies the recipient’s address as unavailable
smtp server permanent error codes
  • 551 — the sending server requires authentication or the recipient’s address is unavailable for the sender
  • 552 — the recipient’s inbox is full, it can’t receive the message
  • 553 — the recipient’s address doesn’t exist, or you spelled it wrong
  • 554 — transaction stopped and failed, no details are available

How is SMTP different from other email protocols?

Email protocols are methods that establish a connection between servers. They use the connection to transfer an email. So an email travels from the sender’s computer to the email client’s server. It then goes to the destination server. The server stores the email for the recipient to access. Or they download the message to their device.

Email protocols differ depending on how they establish the connection between servers. The protocols also differ in how they allow users to access their emails. There are 2 types of email protocols, inbound and outbound:

  • Inbound email protocols allow users to receive and access their emails. POP3 and IMAP are the most popular email protocols of this type. 
  • Outbound email protocols, in turn, relay a message from the sender to the recipient. SMTP relay is an outbound email protocol.

These are further divided into SMTP, POP, and IMAP email protocols. The SMTP is the first protocol used on the journey of an email from a sender to a recipient. 

After the email reaches the server, its next step is the IMAP or POP3 protocol. IMAP allows receiving an email and accessing it from various devices. POP3 allows receiving an email and downloading it from the server to access it offline at any time.

pop3 vs imap email protocols

You can choose which email protocol to use for your email client. Since SMTP is the only outbound email protocol, there’s no other choice for you. But it will ensure safe and worry-free marketing campaigns with transactional emails. This protocol also allows simple data collection and reporting on the campaigns. The data storage is centralized so all team members can access it.

But as for IMAP and POP3, when should you choose which? Go for the one that meets your needs most:


  • centralized storage — access to the mail server from any device or location
  • best for teams with several people
  • higher speed


  • offline access to emails for unstable Internet connection
  • best for individuals
  • uses less email servers space

Running your SMTP server vs. Using a third-party email service: What’s best?

Emails are a great addition to any marketing strategy, so we must manage them well. You have to choose between hosting your SMTP server address and using the service of an SMTP provider or a free SMTP server. Both options have their benefits, but there are some drawbacks to each, too. Let’s discuss what to expect.

Hosting your SMTP server

Running your mail server means you have fewer limits. It gives you complete control over the privacy and security of your mail. But this is both a good and bad thing. You also have to know how to ensure security and troubleshoot problems.


  • You choose virus and spam protection software
  • Full control of outgoing email servers’ privacy
  • No storage fees
  • No email volume limits
  • Hosting multiple domains
  • Non-shared IP address
  • Great customization possibilities


  • Quite difficult to get the hang of it
  • The saved money goes to updating security software
  • Email backups are your responsibility
  • You have to store the backup data in a different location
  • Higher bounce rates and chances to get blacklisted

Using a third-party email provider

Unless you get everything you need from a free plan, you most likely have to pay for the mail server service.

Email providers are usually good at keeping their servers running at all times. Most services have their support teams available round the clock. So you get quite a handful of benefits when trusting your mail to an SMTP service provider.


  • Lots of software providers to choose from
  • Various subscription plans — only pay for what you need
  • Fast, professional troubleshooting and 24/7 support
  • Secure email backups in multiple locations
  • Timely virus protection and spam software updates
  • No need to spend time figuring out how everything works
  • Trustworthy sender’s reputation and low bounce rates


  • Fully relying on the service to troubleshoot issues
  • No control of data protection
  • Limited customization

5 Best SMTP Server Providers

There are many STMP services to set up an SMTP server. We collected the best free SMTP servers, which also offer upgraded plans if you need more features. But here’s a shortlist for convenience.

1. Brevo

Brevo email marketing tool with smtp server

Brevo is one of the most popular email marketing tools. It also includes an SMTP server to send email campaigns and transactional emails.

Along with an automation workflow builder, landing pages, forms, drag-and-drop editors, SMS marketing, live chat, and ads. Read our full Brevo review to learn more about how these features work.

Some free features in Brevo are APIs, real-time reporting, templates, and a landing page builder. The free plan limits you to 300 emails daily. 

Transactional emails start from $15/month for 20K emails. There are 2 subscription plans in Brevo for the marketing platform, too. Starter prices kick in at $25/month for 20K emails. And Business, starting at $65 monthly for 20K emails with marketing automation, ads, landing pages, and multi-user access. 

Get started with Brevo today


smtp2go worldwide email delivery service

SMTP2GO is a reputable email delivery service. It has all the tools to properly set up and manage your outgoing emails. Your email delivery is uninterrupted even during an internet breakdown because of its redundant multi-data center infrastructure.

SMTP2GO has an easy-to-use API, advanced email testing options, and actionable real-time reports. Validate your email with DKIM and SPF. Check how your emails display in 40+ email clients. You can track metrics like bounce rate, open rate, click rate, and unsubscribe rate.

Try SMTP2GO for free and get 1000 emails monthly and 5 days of advanced reporting. The Starter plan costs $10/month for 10K emails, SMS messaging, 30 days of reporting, subaccounts, ticket, chat, and phone support.

Try SMTP2GO for free here

3. MailerSend

Mailersend transactional email service and safe email delivery

MailerSend is a transactional email service with a drag-and-drop editor from the makers of MailerLite. If you’re considering a full email marketing platform, read our complete Mailerlite review. It delivers your transactional emails safely, helping to improve your delivery rate. 

MailerSend offers email verification, transactional SMS, interaction tracking, campaign analytics, push updates, automation, and 10 integrations.

smtp server feature in Mailersend

Sign up for free and get 3,000 emails monthly. 1000 extra emails cost only $1. The Premium plan is $30/month for 50,000 emails, 100 email verifications, premium IP pool management, live chat support, and dedicated IP.

Try MailerSend for free here

4. SMTP.com

smtp.com homepage

SMTP.com is a specialized SMTP email relay service for marketing and transactional emails. They’ve been in business for 20+ years serving 100K+ companies.

The tool has secure authentication, reporting, features to protect your sender reputation, expert support, and 12 integrations.

SMTP.com pricing starts at $25/month for 50K emails, email relay API, detailed reporting, shared IP, email and phone support. 100K emails with a dedicated IP cost $80/month.

Sign up for SMTP.com here

5. Mailtrap

Mailtrap is an email software for devs to test, send, and control emails. Developers can inspect and debug emails in staging with an email sandbox. And send production-ready emails with its API/SMTP Relay. It works well to catch and fix early-stage sending issues with the analytics toolset.

Mailtrap software to create a fake SMTP server

Mailtrap offers a free plan with 500 test emails per month, 1 inbox, and API access. They also have 5 monthly subscription plans. Prices start from $9.99 for the Individual plan with 5000 test emails per month and 5 inboxes.

Wrapping Up: What is SMTP

Let’s recap what an SMTP server is. SMTP stands for Simple Mail Transfer Protocol. The protocol is a set of rules that email clients use to send your emails. SMTP server is an outbound email protocol. Some other email protocols are IMAP and POP3. These are inbound email protocols. 

SMTP servers use commands to communicate with inbound servers. They use error codes to notify of issues. 4.X.X errors are temporary, and 5.X.X won’t resolve without action.

It’s best to rely on free SMTP services for setup if you’re a beginner with low volume. You get more features, quick responses for troubleshooting, and centralized data storage. Some of the best SMTP server providers are Brevo, MailerSend, and SMTP2GO.

SMTP servers, email verification tools, and email testing tools help reduce bounce rates, improve your sender’s reputation, and increase your email delivery rate.

About Sofiia Kasianenko

Sofiia is a freelance content writer with 10 years of experience and a master's degree in journalism. She specializes in media, communications, email marketing, and email protocols like SMTP. Besides writing, she is passionate about creativity, professional growth, fitness, and well-being.

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