When an email message is sent to a different domain than the user’s domain, Simple Mail Transport Protocol (SMTP) ensures the message is forwarded to the recipient’s domain. SMTP relay services provide businesses with a way to use a separate domain and email server when sending bulk email. This provides a way for businesses to send marketing messages to thousands of recipients without having the business domain blocklisted as spam.


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How SMTP Relay Works

Email message transfer is similar to the way regular snail mail works. If you think about the way the post office transfers mail from one location to the other, you can conceptualise the way email message work on the Internet. With snail mail delivery, the sender puts an envelope in a mailbox that a postal worker picks up and delivers to a central office. From the central office, the message is carried by plane or truck to the recipient’s main central office. A postal worker picks up the message from the central office and delivers it to the recipient’s home.

SMTP relay services work in a similar way. The sender’s message is sent to an SMTP server and placed in a virtual envelope. The SMTP server identifies that the recipient’s domain is not the sender’s domain, so relay services send it off to the recipient’s main email server that handles incoming messages. The incoming email server holds the recipient’s email message until the user can retrieve it.

Different protocols are used for incoming email messages such as Post Office Protocol (POP) and Internet Message Access Protocol (IMAP). The difference between these two protocols is that POP holds email until a user retrieves it. Once it’s retrieved, the message is deleted from the server. POP is used mostly with Internet Service Provider (ISP) email servers. IMAP will keep email messages on the server unless the user chooses to delete it. This protocol will synchronise messages between the user’s email client and the server without deleting messages. IMAP is common with business email servers or services such as Google Gmail.

Email servers with SMTP configured perform a few additional steps before sending the message. The server creates a virtual envelope and creates one for each recipient address. It also handles adding headers to the message that can be used to identify other properties of the message.

Authentication is often used with SMTP and its relay services. Because SMTP servers openly run on the Internet and are available to the public, authentication stops spammers from using business servers for spam messages. ISPs and other email services such as Gmail also use authentication on public relay servers to stop spammers.

SMTP takes care of sending messages across the Internet, but what if you want to send bulk emails for your business? Many businesses use bulk email to send marketing messages, notifications to a mass number of users, and many other reasons. An SMTP relay service allows businesses to use their email servers to send bulk email and handles the rules behind opt-out and unsubscribe requirements. Without relay services, the business risks having their domain or internal email servers blocklisted as spam.

Why Is SMTP Relay Important?

Without SMTP, users could not send email messages to anyone outside of their own domain. It would eliminate the usefulness of email messages and online communication. Most email clients seamlessly work with SMTP servers so that the user does not need to manually send messages to the server. Without an email client, users would have to Telnet to an SMTP server and use commands to send an email.

With relay services, businesses can send marketing emails without having their domain blocklisted. Think of SMTP relay services as a way to rent a third-party domain and server to send bulk email on behalf of a specific business. When a business domain or email server is blocklisted, email messages are dropped by a recipient’s email server, even if it’s a legitimate message sent from one sender within the organisation. This can be devastating for business productivity as messages are either never delivered or sent automatically to the recipient’s spam box. SMTP relay services allow businesses to send email without using their own servers or domain so that marketing messages can be separated from internal email communications.

Email client software such as Outlook or Thunderbird provides a graphical interface for users to work with SMTP relay. These GUI programmes provide a way for users to interact with SMTP relay by simply configuring the software to connect to the server. If the server requires encryption over HTTP or authentication, these configurations can be set up in the software. The software takes the recipient’s address, the sender’s address, and the message and makes a connection with the SMTP server.

SMTP relay’s main importance is in its ability to send messages to the intended recipient on behalf of a third-party. No other protocol is responsible for managing outgoing messages, so SMTP is configured across thousands of email servers. Email is one of the most insecure forms of communication over the internet, so many providers have incorporated SSL/TLS with SMTP relay connections. Several other cybersecurity implementations have been added to email communication to help with cybersecurity, such as Sender Policy Framework (SPF), Domain Keys Identified Mail (DKIM), and Domain-based Message Authentication Reporting and Conformance (DMARC).

Benefits of SMTP Relay

Any time a user sends a message to a domain different than their own, the SMTP relay is used. Businesses can use their own internal SMTP servers, but another option is using a cloud-based server. ISPs offer SMTP servers for their users, but businesses must set up email servers that work with their own domain. Third-party services such as G Suite offer an easy way to connect email services with a business domain, but there are several other cloud-based options that make it easier to have business email without maintaining an internal server.

SMTP relay services offer businesses a way to send marketing messages without having their own domain blocklisted. Businesses that must send bulk marketing emails can use SMTP relay services that handle the opt-out options and send messages using the marketing domain. These services shield the business domain from being blocked by end-users and email servers that will immediately drop emails from known spam domains.

Several other benefits of SMTP relay are:

  • Deliver a large volume of messages using SMTP relay servers that deal with bulk email and marketing.
  • Manage opt-out and unsubscribe features to comply with many of the regulations that place guidelines on the way marketing messages are sent. Users must have the option to unsubscribe to avoid being blocklisted.
  • Track messages and identify if they were sent to an inbox and read by the intended recipient using read receipts.
  • Some relay services offer an interface that makes it easier to review marketing messages before they are sent to customers.
  • Ensure that messages are sent across the internet without having an internal business server that handles messages.

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