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What Is an SMTP Relay, and How Does It Work?

Angela Stringfellow

Last updated on

We all use emails on a daily basis. After keying in the recipient's address, the email’s subject, the body of the mail and attaching any files, we hit the ‘Send’ button and the email magically appears in the inbox of the recipient. It’s a deceptively simple process that involves multiple technologies.

You need to have a thorough understanding of the underlying technology if you want to manage your own email server and deliver large volumes of emails (such as transactional emails) on a regular basis.

Understanding Email Technology

For starters, you need to have a basic understanding of email infrastructure and some of the technologies used to deliver emails on a regular basis.

  • Mail server: This is where emails are stored. This can be from a service provider like Gmail or Outlook, but you can set up your own mail servers in the cloud or even on your own computer. In an email communication, the same email will be stored in the mail server of the sender and the recipient.
  • Mail client: This is the application on which emails are viewed. This could be a web application or mobile application. For instance, when using a Gmail account on a web browser, you’re accessing the emails residing on the mail server using the web application mail client.
  • IMAP & POP: IMAP and POP are the common protocols used to access and read emails stored on the mail server. These protocols allow you to access emails from any device, using any email client. IMAP stands for Internet Message Access Protocol and POP stands for Post Office Protocol. The most recent version of POP is POP3.
  • SMTP: SMTP stands for Simple Mail Transfer Protocol. This is the protocol by which emails are delivered from one mail server to the mail server of the recipient.

The technologies mentioned above are the core requirements for email infrastructure to work. The SMTP relay is a layer of technology on top of this infrastructure that increases the speed and deliverability of emails.

What Is an SMTP Relay?

An SMTP relay helps to propagate emails from the sender to the receiver over the internet. The internet is wide and expansive, which makes it difficult to identify the email servers of the recipient while sending an email. This is not a large challenge when you are sending individual emails, but it becomes extremely difficult to deliver emails promptly. More specifically, this becomes a problem when you want to deliver a large volume of transactional emails.

SMTP helps to deliver third-party emails through their servers. In general, services that provide SMTP relay servers have email servers across the globe. They’ll also have the IP addresses mapped for a large number of email addresses, which helps them to route emails quickly and efficiently. SMTP relays are also called smart hosts. If you intend to send a large volume of emails regularly, you should consider using an SMTP relay service.

How Do SMTP Relay Servers Work?

When we send traditional mail or packages, we utilize a logistics infrastructure to deliver the mail from the sender to the recipient. In most instances, we rely on third-party logistics service providers, also called courier services, such as USPS, UPS, DHL and FedEx. SMTP relays for emails function similarly to the way third-party logistics providers work for physical mail.

Third-party logistics service providers operate with a hub and spoke model. For example, imagine you need to send a package from Nashville, TN to Bhatinda, India using FedEx. First, you’ll request a pick-up from the local FedEx office. The package then goes to the distribution center in Nashville, and when the address of the recipient is scanned, FedEx recognizes the mail has to be delivered internationally. It then gets sent to the superhub in the United States which is in Memphis, TN. The transport from Nashville to Memphis could be by truck or air depending on how FedEx sees fit.

The package is transported to the FedEx superhub in India, let’s say, in New Delhi by air. The mail then gets forwarded to the local Bhatinda FedEx office and finally reaches the doorstep of the recipient. This transit of the mail from Nashville to Bhatinda was possible due to the infrastructure capability and knowledge of FedEx. Not only is FedEx able to route the mail, but it also has the ability to reroute mail in case of emergencies such as natural disasters. In countries where FedEx does not have its own network, the courier relies on local logistics providers to route the mail on their behalf.

Similarly, SMTP relays have extensive knowledge of email servers across the internet. They leverage this knowledge to ensure fast and reliable delivery. They also have tie-ups with other SMTP relays for faster and more accurate propagation of mails. SMTP relays route emails to avoid network bottlenecks, limitations imposed by ISPs, and a myriad of other factors.

Wrapping Up

SMTP relays are crucial in ensuring the deliverability of emails. Such service providers have intrinsic knowledge of how to deliver large volumes of emails across the internet with speed and accuracy. You should consider using SMTP relays as part of your email strategy, especially when it comes to delivering time-sensitive emails.

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