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Best Practices

What Is an SMTP Relay and How Does It Work?

By Ken Simpson | 6 minute read

Email can be tricky to get right. From poor email deliverability to IP blocklisting, web hosts can often run into a lot of issues. However, an SMTP relay service can often solve a lot of these problems. Here, we go back to basics; what is an SMTP relay service and how can it improve your business?

What does SMTP stand for?
SMTP stands for Simple Mail Transfer Protocol, the standard way that email is transferred on the internet.

“An SMTP relay service guarantees that sent emails arrive in their recipients’ inboxes.”

What is an SMTP relay service?
In a classic foot relay race, a baton is passed from runner to runner until the baton reaches the finish line. With SMTP, the message is the baton. An SMTP relay is a mail server or “MTA” (Message Transfer Agent) that is directed to hand off your message to another mail server that can get your message closer to its intended recipient – the finish line.

An SMTP relay service is simply an SMTP relay that is run “in the cloud” rather than your own data center. The service receives a message from your server,  queues it up for delivery to its final destination, and then either succeeds in delivering the message or generates an “NDR” (Non-Delivery Report) or “bounce” that is delivered back to the original sender detailing the reasons for non-delivery.

How does an SMTP relay service work?
When you send an email, the email application on your computer or mobile device connects to an SMTP relay and sends the message along with details the relay needs to figure out the next step in the message’s journey. The relay uses the domain name in the email address and the Domain Name Service (DNS) to figure out where the email should be sent. From there, the email may be sent directly to the “MDA” (Message Delivery Agent) of the recipient’s email service. Or, more likely, travel through multiple Mail Transfer Agents operating as SMTP servers before it hits the recipient’s inbox.

SMTP relays may be owned and managed by your company, an email provider like Gmail, or a company like MailChannels.

Why does my company need an SMTP relay service?
If you send email from a server on the internet on behalf of more than one user, chances are you have already run into problems getting email delivered, such as having your server’s IP appear on an IP blocklist. If email delivery has been problem-free, then it’s merely a matter of time before you experience an issue: delivering email is a tough business. And, you may not even know that you’re having email delivery issues, because the worst possible email delivery problem—email that silently hits the junk folder—is rarely noticed by users until it’s too late.

Using an SMTP relay service to send your email relieves you of the burden of dealing with a litany of email delivery issues, from IP blocklisting to reputation problems that cause messages to silently go to the junk folder.

How Our SMTP Relay Service Works

When MailChannels Outbound Filtering receives an email message, several steps are followed before the message is delivered to its intended recipients:

  1. The original sender of the email is identified – for instance, if the message came from a PHP script on your server, we identify the script name and path, and the user who is responsible for it.
  2. The email is scanned by top-notch commercial spam and phishing content filters to identify malicious content.
  3. We check the sender’s reputation out using our internal reputation system.
  4. We check whether any of the recipients of the email are actually “spam traps” (see below for details), which is an indicator of a severe problem.
  5. We check the reputation and make-up of the sender’s email domain (i.e. the part after the “@”) – for example, domains that try to impersonate well-known brands like PayPal™ are flagged.
  6. If any of the above steps indicate a severe problem, then the email is immediately rejected. Otherwise, we might take note that the sender is acting suspiciously and be more vigilant when we see more emails from them in the future.
  7. If everything checks out, then we attempt to deliver the email to its intended recipients by contacting the mail servers associated with the email recipients.

“Unlike other SMTP relay services, MailChannels Outbound Filtering goes a step further and also allows you to receive automated notifications of problems with your email traffic.”

Even after the email is delivered, our job is just getting started. MailChannels Outbound Filtering analyzes the response messages generated by the receiving email servers and classifies them to see what the receiving email servers think of the email. A response such as “550 5.7.1 Message blocked due to spam content” would indicate that the receiver thinks the message is spam. This information is fed back into our internal reputation system to potentially rate limit or block the email sender when they try to deliver more emails in the future.

In addition to processing the response messages, MailChannels Outbound Filtering also digs into “feedback loop” data generated when email recipients click the “Spam” button in their email client. If email recipients complain about an email that is sent by MailChannels Outbound Filtering, we automatically attribute those complaints to the email sender who originated the message; this feedback helps us identify senders who might be sending spam and other unwanted junk so that we can protect our reputation with email receivers.

Giving Feedback to You and Your Team
When MailChannels Outbound Filtering rejects an email—as with any SMTP relay service—the sender receives an NDR to let them know the email could not be delivered. But unlike other SMTP relay services, MailChannels Outbound Filtering goes a step further and also allows you to receive automated notifications of problems with your email traffic.

For example, if MailChannels Outbound Filtering detects that a user is sending spam, a notification is generated and sent to you automatically to let you know about the spamming activity. This notification allows you to follow up with the user, whose account may have been hacked. The MailChannels Outbound Filtering notification system is sophisticated, yet easy to use, and can notify you either by email or through a webhook API, enabling automated processing of notifications by your own scripts.

* This blog post was revised in June 2018

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