Ever wondered how your marketing emails make their way to thousands of inboxes without any issues? Or why some of your messages land in spam while others don't? The secret lies in a powerful tool called SMTP relay—the often-forgotten-behind-the-scenes hero of email delivery.
Whether you're sending out newsletters, transaction confirmations, or password resets, the reliability of your email delivery can make or break your customer relationships. That's where SMTP relay services come into play.
This post provides a comprehensive explanation of SMTP relay servers. You’ll learn the following:
Simple Mail Transfer Protocol (SMTP) is a straightforward process that allows users to send, relay, and monitor outgoing emails. An SMTP relay service—also commonly known as an SMTP gateway service—takes this process a step further by allowing you to send more transactional and marketing emails.
Instead of managing your program, you can trust the SMTP relay service to manage and maintain the SMTP server for you.
But why is it crucial to have an SMTP relay service and how does it work?
Hosted on the cloud and accessed on a web browser, peer-to-peer email isn’t sufficient for specific types of mass email campaigns and programs. Many businesses that send out bulk emails, such as newsletters or automated messages prompted by user interaction, need more tailored outgoing email solutions. Utilizing an SMTP relay server for email distribution provides just that.
When an email service provider (ESP) tries to send an email, it connects to the configured SMTP server to communicate. Once the ESP can communicate with the configured server, it can deliver the email message through an SMTP relay server.
SMTP relay servers work similarly to snail mail—the server essentially acts as the post office.
Imagine that you have to send someone a physical letter. You write down addresses for yourself and the recipient on an envelope. After you drop it off at your local post office, the post office then determines which other post office is closest to the recipient and directs the letter there.
When you send an email, the message includes the recipient’s destination address and your return address information—for example your email address, IP address, and time stamp.
Your local server uses this information to send your message to the recipient’s server for successful delivery.
SMTP mail relay is the process of transferring an email from one server to another for delivery.
For example, if you work for company A and send an email to someone at company B, you connect to your company’s SMTP server. Then, your company’s SMTP server delivers your email to company B’s server. One server accepting an email from another server is relaying.
If you send an email to someone with an email address at the same domain as yours, there isn’t a second server involved. This isn’t relaying. It’s like sending a physical letter to someone in your town—your local post office receives and delivers it without involving another post office.
SMTP mail relay routes emails through a trusted third party to help deliver high volumes of email. It specializes in sending large batches of emails, such as newsletters, or automatic transactional emails, such as delivery confirmations and password resets.
When an SMTP relay server is public, there’s only so much a business can do before having to search for more costly options. A public SMTP relay server is free to use but has limited capabilities when sending mass marketing emails.
To counter spam, many internet providers put a cap on how many SMTP relays it can conduct per day.
Because of bulk emailing needs, businesses often exceed the public SMTP relay limit and require enterprise-level email-sending features. An SMTP relay provider can help businesses deliver large amounts of emails without them getting mislabeled as spam.
SMTP authentication, or SMTP AUTH, is an extension that grants clients access to an SMTP server through an identification mechanism. Authenticated SMTP relay is the process of verifying the sender’s credentials before processing and relaying the email.
Open mail relays don’t have the same security requirement and are more prone to misuse. SMTP authentication helps protect your server from getting spammed by malicious emails while allowing you, a trustworthy user, to receive and send emails securely.
The SMTP server address is an important piece of information when configuring your email client or application to send emails. It's essentially the domain name or IP address of the server that handles outgoing mail.
Here's what you need to know about SMTP server addresses:
Format: An SMTP server address typically looks like "smtp.domain.com" or "mail.domain.com". For example, Gmail's SMTP server address is "smtp.gmail.com".
Finding your SMTP server address: Your email service provider usually gives you this information. You can often find it in your email client's settings or your provider's documentation.
Common SMTP server addresses: Here are some examples for popular email providers:
Gmail: smtp.gmail.com
Outlook/Hotmail: smtp-mail.outlook.com
Yahoo Mail: smtp.mail.yahoo.com
Port numbers: SMTP server addresses are often associated with specific port numbers. Common SMTP ports include:
Port 25: Standard SMTP (usually blocked by residential ISPs)
Port 465: SMTP with SSL encryption
Port 587: SMTP with TLS encryption (recommended for most users)
Security considerations: When setting up your SMTP server address, always use encryption (SSL/TLS) to protect your email communications. This is especially important when sending sensitive information.
Remember: if you're using an SMTP relay service like Twilio SendGrid, you'll use their SMTP server address instead of your email provider's. For SendGrid users, the SMTP server address is "smtp.sendgrid.net".
Managing your email server for peer-to-peer email communication is relatively straightforward because most emails only go to a limited number of recipients.
But what if you want to send a marketing newsletter to tens of thousands of subscribers? What if you have a web app that sends notification emails prompted by your recipients’ activities?
Those emails aren’t as unique and are often labeled as spam. This is where an SMTP relay service can help.
If you use your primary peer-to-peer email server to send bulk emails, your delivery rate will suffer. And you may clog your bandwidth, potentially delaying the sending and receiving process of peer-to-peer emails.
Constantly monitoring your email server to keep crucial emails out of the spam folder is an expensive and time-consuming task.
As your business grows, so do your email-sending needs.
An outbound SMTP relay service ensures that common delivery mishaps won’t affect your email campaigns—and SMTP relay providers like Twilio SendGrid can help you get started.
Using an SMTP relay service can streamline your email delivery process, but like any technology, you might run into some challenges. Here are some troubleshooting tips for the most common issues such as IP blocklisting, high bounce rates, and delays in email delivery.
IP blocklisting occurs when your sending IP address is added to a blocklist used by email providers to block spam. This can severely affect your email deliverability. Here's how to fix it:
Monitor your IP reputation regularly: Use tools like SenderScore, Talos Intelligence, or MXToolbox to check your IP reputation. These services can alert you if your IP gets blocklisted.
Warm up your IP: If you’re using a new IP address, start by sending a small volume of emails and gradually increase the amount. This helps to build a positive sending reputation with ISPs.
Follow email sending best practices: Ensure your email lists are clean and up-to-date, avoid spammy content, and use double opt-in methods for subscriber lists to maintain list quality.
Request de-listing: If you find your IP address has been blocklisted, contact the blocklist operator and request removal. Be prepared to explain how you’ve remedied the issue that caused the listing.
Bounce rates can spike due to invalid email addresses, server issues, or aggressive filtering by ISPs. Here's how to fix it:
Validate email addresses: Use email validation services before adding addresses to your mailing list to ensure they are correct and active.
Segment your lists: Regularly clean your email list by removing inactive subscribers who do not engage with your emails over a significant period.
Review email content and frequency: Ensure your content is relevant and not too frequent to avoid recipient fatigue. Check for triggers that might cause spam filters to block your emails.
Analyze bounce messages: Pay attention to the bounce messages you receive as they can provide clues on why emails are not being delivered.
Delays can occur due to network issues, server overload, or configuration errors. Here's how to fix it:
Check server load: Ensure your SMTP server is not overloaded with requests. If persistent delays occur, consider scaling up your server resources.
Optimize email size: Large emails with high-resolution images or attachments can slow down delivery. Optimize these elements to speed up the process.
Review configuration settings: Incorrect settings in your SMTP server can cause delays. Verify your configuration settings are correct for timeouts, retry intervals, and maximum message sizes.
Contact your SMTP service provider: If issues persist, contact your SMTP relay service provider. They can provide insights into potential network issues or configuration errors that are not immediately obvious.
An SMTP relay service is a third-party application that allows you to send large volumes of email by routing your emails through their servers. This service acts as an intermediary to ensure that your emails reach their destinations reliably and securely.
SMTP relay services help improve deliverability by managing the reputation of the sending IP addresses, implementing proper authentication protocols (like SPF, DKIM, and DMARC), and handling bounces and feedback loops with ISPs.
A public SMTP relay server is typically free to use but may have limitations in terms of sending volume and is generally less secure. A private SMTP relay server is a paid service that offers greater control, higher sending limits, enhanced security, and better deliverability.
No, they are different. SMTP relay refers to the process of transmitting emails from one server to another. SMTP authentication, on the other hand, is a protocol used to authenticate the credentials (like username and password) of the sender to prevent unauthorized usage of the SMTP server.
Choosing the right SMTP relay service depends on several factors, including your email volume, budget, specific features like analytics and reporting tools, the level of customer support provided, and compliance with security standards.
Twilio SendGrid’s SMTP relay services are highly scalable, with no hardware or software costs associated. Our features adapt to your business needs and allow you additional capacity if needed.
SendGrid also provides various secure SMTP hosting and relay services for transactional email and enterprise jobs. Our transactional email delivery integrates with multiple developer frameworks and comes with template libraries and code samples.
Not only are these efficient and cost-effective, but our enterprise email solutions also provide a holistic messaging delivery platform.
With SendGrid’s enterprise email management, you’ll have access to real-time analytics and reporting tools, dedicated account management, and a support team to help with any inquiries.
