SMTP Relay Service 101 [Back to Basics]


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Email relay handoff imageMost people are familiar with peer-to-peer email hosted via the cloud and accessed via a web browser. However, this type of managed email is not sufficient for specific types of email broadcasts. Many businesses, which send out bulk emails, like newsletters, or automated emails triggered from user interaction, need more tailored outgoing email solutions.

You may have heard the term SMTP Relay Service, especially if you’re looking for an email provider for newsletter or transactional email delivery, but what exactly is that? In plain English, it is a service that delivers outgoing email and does not accept incoming email. But what exactly is important about only delivering outgoing email, and how does it work?

What is SMTP?

Cloud Email Delivery imageSMTP stands for Simple Mail Transfer Protocol, and most email services in the world use SMTP to send email. When an email service tries to send an email, it connects to the configured SMTP server and communicates to it using the SMTP protocol. Once it can communicate with the configured server, it can then deliver the email message.

In simpler terms, SMTP relay is much like how snail mail works, with the server acting as the post office. In order to send a paper letter, you must address an envelope and fill in your return address. Your local post office then determines which local post office is closest to the recipient, and then directs the letter there to be delivered. When you send an email, the message includes the recipient’s destination address as well as your “return address” information – i.e., your email address, IP address, and timestamp. Your local server then sends your message to the recipient’s server to be delivered.

What is Mail Relay?

Mail relay is the process of transferring an email from one server to another for delivery. An example of this is if you work for Company A and send an email to someone at Company B, you connect to your company’s SMTP server which then relays your email to the server owned by Company B. One server accepting an email from another server is called “relaying.”

If you were to send an email to a person with an email address at the same domain as yours, there would not be a second server involved, so it is not considered relaying. It’s much like sending a paper letter to someone in your town – your local post office receives and delivers the letter without involving another post office.

What is an SMTP Relay Service?

SMTP Relay is a service that routes email through a trusted 3rd party to deliver your email. It most often specializes in sending large batches of emails (newsletters) or for automatic transactional emails (delivery confirmations, password resets, etc). Many internet providers put a cap on how many SMTP relays it can conduct per day to combat spam. Businesses often exceed this limit and need enterprise level email sending features. An STMP Relay provider can help businesses deliver large volumes of email without getting mislabeled as spam.

Why is SMTP Relay Important?

Transactional notifications imageManaging your own email server for peer-to-peer email is pretty straightforward because most emails are unique and are only sent to a limited number of recipients. But, what about when you want to send out a marketing newsletter to tens of thousands of subscribers? Or what if you have a web app that sends out notification emails triggered from your users’ activities? Those emails aren’t as unique and often get mistakenly labeled as spam. An SMTP relay provider like SendGrid, can help.

If you were to use your primary peer-to-peer email server to take on large bulk mailing jobs, your delivery rate would suffer and you would inevitably clog your bandwidth and potentially delay sending and receiving peer-to-peer emails. Constantly monitoring and configuring your email server to keep messages out of the spam folder can be expensive and very challenging. Then, as your business grows, so does your email. Mail server upkeep becomes even more complex and costly.

Outsourcing to an SMTP relay provider is a great way to manage large outgoing email jobs and automated emails without negatively affecting your everyday email communications. SendGrid’s SMTP services are scalable and allow you to add additional capacity as your needs grow.

SendGrid SMTP Relay Services

SendGrid provides a variety of SMTP hosting and relay services for transactional email and enterprise jobs. SendGrid cloud solutions use the world’s largest email delivery infrastructure and are reliable, scalable, and measurable.

Transactional email delivery is easy to integrate with multiple developer frameworks, and comes with template libraries and code samples. Transactional email management is highly scalable and there are no hardware or software costs associated with it.

Enterprise email solutions are also highly scalable and provide a complete messaging delivery platform. Enterprise email management comes with real-time analytics and reporting tools and customers are assigned a dedicated account management and support team to help with any needs.

For more information on SMTP relay services, the ins and outs of email deliverability, and whether you should build your own email infrastructure or outsource it to a third party, visit our Resources section.


Carly Brantz is a veteran in the email deliverability space working to make email simple and easy for developers by regularly writing whitepapers, research briefs and blog posts about email, technology and industry trends.

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