On the surface, sending emails seems like a straightforward process. You enter the recipient’s email address, type your message — maybe add an emoji or seven — and hit “send.” A few seconds or minutes later, the email arrives in the recipient’s inbox. Magic, right?
Not quite. Gmail, Outlook, and other email clients rely on Simple Mail Transfer Protocol Servers (SMTPs). So, what is a SMTP server? This infrastructure allows email clients to send and receive emails. It’s the reason why you can send messages to any valid email address around the world.
Understanding how SMTP servers work will allow you to increase email deliverability and improve your marketing campaigns. This guide provides an in-depth explanation of SMTP servers and their role in email marketing. It also highlights the benefits of setting up a dedicated SMTP server for business communications.
What is a Simple Mail Transfer Protocol (SMTP) server?
Simple Mail Transfer Protocol is an application that transports emails from one inbox to another. This protocol — or set of rules — allows email clients to send, relay, and receive messages from any email address.
What is the function of a SMTP server? This tool has several roles in email communications, including:
- Relaying emails between clients
- Managing email queues for bulk emails
- Verifying that emails get delivered to valid addresses
- Generating error codes if the server can’t deliver emails
- Keeping sent emails secure
- Documenting all email transfers
- Filtering spam and junk mail
Without an SMTP server, your email would disappear into the void and never reach its recipient. You wouldn’t get a notification that the message wasn’t delivered, so you wouldn’t realize that there’s an issue — at least, this is, until you notice that no one responds to your emails.
SMTP servers, relays, and proxies
People often use the terms SMTP servers, relays, and proxies interchangeably. However, these applications have different functions in the email communication process.
An SMTP server receives incoming emails and delivers them to inboxes. It’s like an air traffic control tower directing planes as they take off and land.
An SMTP relay is an intermediate application that pushes emails between SMTP servers. It’s often used to transfer messages across different email providers. To stick with the airport metaphor, an SMTP relay is like a connecting flight hub that lets passengers switch from an American Airlines flight to a Delta flight.
Finally, an SMTP proxy scans incoming messages and accepts or denies them based on specific filters. For example, an SMTP proxy could deny emails with malicious code. It’s like a security checkpoint at the airport that prevents dangerous items or people from entering a secure area.
Technical details
SMTP servers may seem complicated, especially if you’re not a computer science expert. But don’t feel intimidated. These applications operate in predictable and simple ways.
How does an SMTP server work?
Let’s break down the function of an SMTP server with a hypothetical scenario:
- You log into your Gmail account and write a marketing email promoting your business’s new kayaking gear.
- You enter a customer’s email address (kayakfan23@outlook.com) in the “To” field and click “Send.”
- The Gmail client automatically connects with its SMTP server through an SMTP port and sends it the email. This exchange is known as an SMTP handshake.
- The SMTP server verifies that the email has the correct sender, recipient, and domain.
- Gmail’s SMTP server passes the message to the server associated with the recipient’s email domain.
- The receiver’s SMTP server scans the email for viruses and passes it through a spam filter. If the message passes these checks, the SMTP server delivers it to the recipient’s inbox.
- The customer opens the email and decides to buy the new kayaking gear after reading your persuasive email content.
This process may seem extensive, but these steps only take seconds or minutes if the email has no issues.
Basic SMTP commands
Email clients use SMTP commands to communicate with and direct servers. Here are a few essential commands:
- HELO: The email client uses this command to begin the conversation with the server. It also shares its domain name so the server can identify it.
- AUTH: The client authenticates itself to the server.
- MAIL FROM: This command begins the mail transfer process. The sender’s email address follows the command.
- RCPT TO: The client specifies the recipient’s email address.
- DATA: The client requests permission to share the message’s attachments, subject line, and other contents.
- QUIT: This command clears the data and shuts down the connection between the client and server.
SMTP errors codes
You may ask, “What happens if a SMTP server is shut down or has a problem?”
If the SMTP server cannot deliver the message to the recipient, it sends the email client a three-digit error code. Analyzing these codes can help determine if your email is experiencing a hard or soft bounce.
Common SMTP error codes include:
- 450: The server can’t deliver the message due to a temporary routing issue. This error often occurs if a spam filter rejects the message.
- 452: The receiving server doesn’t have enough storage space to receive the message.
- 500: This generic error message signals that the server doesn’t recognize the client’s SMTP command.
- 501: The command has errors that prevent the server from executing it. You may get this error if you enter an invalid email address.
- 541: The receiving server has flagged the message as spam or placed your email address on a blocklist.
Understanding these errors can help you troubleshoot your emails and correct issues before resending the message.
SMTP authentication methods and security protocols
SMTP servers use authentication methods to verify that the client has permission to send emails. These security mechanisms prevent suspicious accounts from spending spam throughout a network.
Here are a few common authentication methods:
- PLAIN and LOGIN: The client sends their username and password.
- CRAM-MD5: The server sends the client a challenge, and the client must use its password to solve the riddle.
SMTP servers also use various security protocols to protect email users from spam and malicious messages. Popular security protocols include:
- Secure Sockets Layer/Transport Security Layer (SSL/TLS): HTTPS encrypts email messages between clients and servers.
- StartTLS: This protocol uses authentication algorithms and encryption to protect transfers between clients and servers. The recipient’s server decrypts the data so they can read the message.
- DomainKeys Identified Mail (DKIM): The sender digitally signs the email, and the recipient’s server uses this signature to authenticate their identity.
SMTP server address
What is a SMTP server address? This term refers to the domain name or IP address of the SMTP server that sends the message. For example, iCloud’s SMTP server address is smtp.mail.me.com. You can find this information by opening the account settings in your email client and navigating to the outgoing mail settings.
SMTP IP address
What is a generic IP for SMTP server settings in an email? There is no universal IP address, but some email clients provide the sender’s IP address in the message header.
SMTP server domain
What is a SMTP server domain? It’s the name of the SMTP server that receives outgoing mail. It appears after the “@” symbol in the email address.
Email infrastructure
Businesses can choose between local and cloud-based SMTP servers. These servers have a few key differences.
Local SMTP servers:
- Housed on the business’s infrastructure
- No email volume limits
- Track delivery information for every sent email
- Can fully customize the email and security settings
Cloud-based SMTP servers:
- Owned by a third party, such as Apple, Google, or Microsoft
- Require little to no maintenance on the user’s part
- Higher email delivery rates
- More affordable than building a local server
Running your own SMTP server gives you more control over your emails. But this option is more vulnerable to power outages, spam filters, and other email deliverability problems.
Many businesses prefer the convenience and security of cloud-based SMTP servers. But you’ll need to follow your chosen provider’s terms and conditions. These policies may limit your email volume and the type of content you send.
You can avoid some of these issues with cloud-based servers by using an SMTP relay service. This application delivers outgoing emails in bulk, improving scalability.
Integration and setup
Follow these steps to set up an SMTP server for your business needs:
- Select a server platform, such as Mac or Linux.
- Install free or paid SMTP server software on your platform.
- Configure the domain name, security settings, spam filters, and other settings according to your business needs.
- Integrate the SMTP server with your preferred email clients. Popular email clients include Comcast, Gmail, and Microsoft Outlook.
- Use a plugin to integrate the SMTP server with WordPress sites and configure the settings.
- Configure DNS records for your domain name.
Choose an SMTP provider that offers thorough documentation for their servers. These resources will help you understand your chosen service and configure the settings properly.
Don’t panic if you encounter issues while setting up your SMTP server. This checklist will help you troubleshoot errors:
- Reset your internet connection.
- Verify that you’ve configured the server name and port number correctly.
- Review the server logs for error messages.
- Use a different SMTP port.
Alternative email protocols
SMTP isn’t the only method to transmit emails. Here are three alternative ways to send and receive messages.
IMAP
The Internet Message Access Protocol (IMAP) only receives messages instead of sending them. This application retrieves an email from the client and saves it on the server as a temporary file. The server updates the email status immediately when the user opens or deletes it.
POP
Post Office Protocol (POP) retrieves an email from the client and downloads it onto the user’s local device. This method allows recipients to read their messages offline.
Essentially, SMTP pushes emails between the sender’s and recipient’s servers, and POP downloads those messages for the receiver.
MTA
A Mail Transfer Agent (MTA) is part of the email communication ecosystem. It transfers received emails from the SMTP server to the Mail Delivery Agent (MDA). From there, the user’s inbox can retrieve the messages with IMAP or POP.
SMTP in practice
Businesses can use SMTP servers to deliver effective email marketing campaigns. These applications allow you to send mass emails to your mailing list.
Follow these best practices when sending bulk emails via SMTP:
- Choose an SMTP sender with high email deliverability rates.
- Comply with the CAN-SPAM Act.
- Use authentication protocols like SSL and DKIM to avoid getting blocked by spam filters.
- Segment your email list and only send relevant content to customers.
- Build your email list organically instead of using a purchased list.
You can also use email marketing software to send mass emails without using SMTP directly. For example, Constant Contact offers hundreds of templates that you can use to design mobile-responsive emails. The platform sends marketing emails through its own servers to improve deliverability and reduce bounce rates.
SMTP history and evolution
In 1982, Jon Postel and colleagues published RFC 821, which first defined SMTP. This protocol revolutionized digital communication by forwarding emails across different clients and networks.
During the 1980s, organizations typically used local SMTP servers to relay external emails. As email usage grew, companies began relying on third-party SMTP servers, such as Postfix and Oracle Communications Messaging Server.
SMTP also contributed to significant developments in email communication. For example, SMTP servers pioneered authentication protocols to prevent spam.
Free SMTP services
What is a free SMTP server? These third-party SMTP services are free to use. Popular free SMTP services include:
- Brevo
- Gmail
- Mailgun
- Postmark
- SendGrid
Most providers share free step-by-step guides to configuring these services on their websites.
Future outlook for SMTP
Cybersecurity and data privacy concerns pose new challenges for email marketers. Future SMTP protocols may address these issues with artificial intelligence and blockchain technologies. These advancements can improve email deliverability and improve security.
Share your message with SMTP
SMTP servers are the backbone of modern email communication. They transmit marketing emails and other messages between senders and receivers across the internet.
Expand your knowledge by searching for SMTP tutorials on YouTube and social media platforms. Additionally, Udemy offers affordable or free online courses on email servers.
