An SMTP server is software that sends, receives, and relays email messages using the Simple Mail Transfer Protocol (SMTP) to transfer information from one mail server to another across the internet. Acting as the delivery tool for digital mail, SMTP servers process and deliver single, transactional, and bulk emails securely, enabling the billions of daily email communications that businesses and individuals rely on.
There are dozens of ways to communicate digitally today, yet email still stands the test of time. Email remains a prevalent way to communicate for businesses and individuals alike. An estimated 333 billion emails are sent daily, and that number is expected to grow to 376 billion by 2025. For most, emails are as simple as composing a message and clicking "send," but the process behind sending emails is much more complex. For example, transactional email templates show how specific user actions trigger automated communications.
The technical term for email is 'electronic mail,' which has a rich history in the development of networked messaging systems and the evolution of communication protocols.
Behind the scenes, there are multiple systems, servers, and protocols that help deliver your emails. A critical email protocol known as SMTP helps set everything in motion and is at the core of sending emails. SMTP can be thought of as a digital postal service, ensuring reliable delivery of your messages across the internet. SMTP operates over the transmission control protocol (TCP), which provides dependable message delivery between servers. Email services and smtp services are essential components of the email infrastructure, handling the transmission, protocols, and reliability of email delivery. What is an SMTP server, and how does it work? We define what it is, how it works and discuss the pros and cons of running an SMTP server on your own versus using a third-party service.
Defining Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP) is a standard method for transferring information via email, also known as an “email protocol.” The SMTP protocol defines the rules for email transmission, specifying how email servers communicate and handle the transfer of messages. Email protocols are sets of rules used explicitly for sending email messages from one email account to another via the internet. Email protocols allow different email clients (Gmail, Outlook, Yahoo, etc.) and email accounts to exchange information easily.
SMTP is one of the most common email protocols and is the only dedicated protocol for sending emails. Email clients rely on SMTP to push messages from a sender account to a receiving account. Mail clients use standardized SMTP commands to communicate with servers, initiate connections, authenticate, and route messages.
Proper configuration of SMTP settings and the use of correct SMTP ports are essential for successful and secure email delivery.
What Is an SMTP Server?
Now that we’ve defined what Simple Mail Transfer Protocol (SMTP) is, we can discuss SMTP servers. A server is software that provides a service to other computer applications within a network. An SMTP server sends, receives, and relays email messages using the internet to transfer information from one server to another. An smtp email server often acts as the outgoing mail server responsible for sending emails.
SMTP servers act as the delivery tool that processes and sends emails from one destination to another. SMTP servers can deliver single, transactional, and bulk emails securely within minutes. The email transmission process is initiated by a mail user agent (MUA), which is the client software used by end users to compose and send emails. The MUA acts as an smtp client, and smtp clients establish connections with the smtp email server to begin the sending process. Mail submission agents and mail transfer agents (also known as message transfer agents or mail agents) work together to relay and deliver emails through the SMTP infrastructure.
How Does SMTP Work?
SMTP can be a complex protocol, but we’ll explain the technical rules, commands, and sending processes without getting too granular.
SMTP servers are used over common email providers because SMTP are dedicated servers that allow scaling. The first step is to set up the SMTP server, which will enable email clients like Gmail, Outlook, or Apple Mail to communicate with your server. The SMTP session begins when the client sends the HELO command to the server to initiate communication. With the communication stream open, you are now ready to send emails.
Once you’ve sent an email, the email client will open an SMTP connection to your server. The sender SMTP (the SMTP client) establishes a TCP connection with the server and uses the SMTP envelope to specify the sender’s email address and recipient’s email address.
The SMTP server then uses the internet to locate and talk to a domain name server (DNS). The DNS will provide the server information to help the email reach the right recipient. The server performs a mail exchange lookup to identify the recipient’s domain and locate the recipient’s SMTP server. The SMTP server and the DNS work together to locate Mail Exchanger (MX) records such as the sender’s email address, the recipient’s email address, and the email’s content.
If the sender and receiver’s domains are the same, the SMTP server will deliver the email instantly. If the domains differ (for example, you’re sending an email from a Gmail account to an Outlook account), the DNS will need to identify the recipient’s domain to ensure the message reaches the correct server. Once this information is shared with the recipient’s SMTP server, that server will take over to complete the delivery and send the message to the recipient’s inbox.
Errors can occur when email domains and accounts change, or when other technical issues arise. Emails may bounce, be blocked, or get reported as spam. Internet service providers may block certain SMTP ports to prevent spam, and SMTP AUTH is used to authenticate users and prevent unauthorized relaying.
SMTP relay services route emails through other SMTP servers to ensure successful message transfer across different domains. This is why it is essential to have an IT team to help with technical issues, and why marketing teams should always scrub and “clean” email lists to help prevent errors and bounces.
Overall, the SMTP server coordinates all these components -- DNS lookups, authentication, and relay -- to ensure reliable email delivery.
SMTP Server Security
Securing your SMTP server is critical. Without proper safeguards, an SMTP server can become a target for unauthorized access, spam, and phishing attacks -- putting sensitive information and your sender reputation at risk.
Authentication. SMTP authentication ensures that only authorized users can send emails through your server. Common methods include LOGIN, PLAIN, and CRAM-MD5, which require users to verify their identity before the SMTP server will process outgoing mail. Enabling SMTP authentication prevents spammers from relaying messages through your infrastructure.
Encryption. Configuring your SMTP server to use Transport Layer Security (TLS) protects email messages in transit. Even if someone intercepts the data, TLS encryption keeps the contents unreadable. Most reputable email notification services and SMTP providers offer TLS as a standard feature.
Spam and virus filtering. SMTP servers should implement virus scanning and spam filtering to detect and block harmful attachments before they reach the recipient’s inbox. These tools safeguard both your users and your network.
When choosing an SMTP service provider, look for features like SMTP authentication, TLS encryption, spam filtering, and delivery status notifications. These measures protect your outgoing emails and help you monitor the health of your SMTP server.
Running an SMTP Server — Use Your Own or a Third-Party Service?
Now that you're ready to run an SMTP server, the big question is, do you use your own, or a third-party service? There's no “right” answer. It truly depends on your needs and the resources you have. Let's start by looking at what it would take to run your own SMTP server.
Running Your Own SMTP Server
Running your own SMTP server can have many benefits, such as no sending limits. Some third-party providers will put caps or limitations on how many emails you can send per day, week, month, etc. With a dedicated SMTP, you can send all the emails you want. Another significant aspect of having your own server is having full access to monitor all delivery information. This can help you discover error codes and help you improve your email methodology. You also remain in complete control of your email lists, which is important for preserving private data from your company and your customers.
Though there are many benefits to running your own SMTP server, you should first consider whether or not you have the resources to accommodate it. Not only will you need someone knowledgeable to set it up, but SMTP servers need to be monitored regularly. For example, you'll likely need a dedicated team to monitor and maintain the server, which can be costly.
Another thing to consider is that running your own SMTP server will require it to be local. When you have a local server, it can be vulnerable to power and connectivity issues in your area. You'll need backup servers and other failsafe protections in place, plus a staff member with technical knowledge to fix things when they do fail.
Lastly, deliverability and security issues can arise. Even with a great team, there are still challenges preventing spammers from accessing your server. You'll need to have protection plans and processes in place to detect unauthorized spam and adjust strategies as needed. However, if you have the resources to set up and maintain your own SMTP server, the benefits could make this a good option for you.
Using a Third-Party SMTP Service
A third-party SMTP service is an excellent option for companies that lack the staff resources or simply don't want the hassle of running their own SMTP server. Popular providers like Amazon SES, SendGrid, and Mailgun take care of the setup, monitoring, and maintenance of the SMTP server on your behalf. These services already have expert teams for technical support, security, and failsafe plans.
Third-party SMTP providers often include high security and IT support standards as part of the service. Most are billed at a fixed rate, so you shouldn't have to worry about unexpected costs. These companies also know the ins and outs of ISPs and mailbox providers, ensuring reliable deliverability from their SMTP servers without trial and error on your part. For a broader comparison, see our roundup of the best transactional email services for developers.
Using a third-party SMTP service can be less hassle and more secure, cost-effective, and reliable. However, a drawback for some is that you're not in complete control of the SMTP server. You have to rely on others to notify you of issues, relay information, and trust that they're following best practices. You also have to share your company information and customer email lists with them.
Dealing with their terms and conditions can also be a pitfall. As we mentioned above, some SMTP service providers have caps on how many emails you can send in a specific time frame. When hiring an SMTP service, be sure to check for limitations and read reviews to find a provider with superior customer support and domain expertise.
Conclusion
An SMTP server is the backbone of every email you send -- from one-off messages to high-volume transactional notifications. Understanding what an SMTP server is, how it routes mail through DNS lookups and relay, and how to secure it with authentication and TLS gives you the foundation to make smart infrastructure choices.
If you have the resources, running your own SMTP server offers full control over deliverability and data. If not, a third-party SMTP service like Amazon SES, SendGrid, or Mailgun can handle the heavy lifting. Either way, a well-configured SMTP server is essential for reliable email delivery.
For teams that need email alongside in-app, push, and other channels, MagicBell provides a unified notification platform that integrates with any SMTP server or third-party provider -- so you can reach users wherever they are.