Our goal at SendGrid is to make sure that we’re providing our customers, potential customers, and industry friends with the information they need to be successful senders. A big part of that is making sure we’re all on the same page when it comes to some tricky email terminology.

We’ve done our best to demystify some terms that might give new…and veteran senders alike some pause. So what were our most popular email terminology definitions (in descending order) from 2013?

Drumroll please….

  1. SMTP
  2. DKIM
  3. Sender ID
  4. Feedback Loops
  5. Email Throttling

Here’s a Cliffs Notes version of what each of these email heavyweights means. For a full breakdown, read through to the full posts.


SMTP 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.

Read the full article here.


DKIM (DomainKeys Identified Mail) is an email authentication method developed by Yahoo! that checks an encrypted “key” embedded in each email sent against a list of public records to positively confirm the identity of the sender. In short, it helps ISPs identify the good mail and aids in preventing malicious email from getting through. It pretty much works the same way IP reputation does, except it works with your domain name. This is actually better for brands because if you switch email service providers, you can take your domain reputation with you.

Read the full article here.

Sender ID

Sender ID Framework is an email authentication protocol designed to keep spammers out of the inbox. It’s pretty similar to Sender Policy Framework (SPF), but with one main difference; Sender ID verifies sender identity based on the Purported Responsible Address (PRA) domain using the From:or Sender: header fields. For those a little less technical, it’s simply a different way to identify the legitimacy of a sender.

Sender ID was developed by Microsoft to help prevent spammers from duping unsuspecting customers into downloading malware or giving away their personally identifiable information. By authenticating your email streams with Sender ID, you are helping mail receivers sort through the barrage of spam they receive on a daily basis. There had been some talk that Sender ID was dead, but Sender ID continues to provide important benefits to both senders and receivers.

Read the full article here.

Feedback Loops

Spam complaints have a huge impact on your sender reputation, so wouldn’t it be helpful to know who’s reporting your email as spam? You can with feedback loops. Feedback loops were once a tool for ISPs to monitor abuse from their networks, but now, they are also available to senders who would like to use the information to perform list hygiene and to gain insights into what may be negatively affecting their email program.

Read the full article here.

Email Throttling

Some ISPs limit the amount of email they accept from a particular sender during a specified period of time. If you try to send email above their acceptable threshold, they will reject your email resulting in a high number of bouncebacks. This is email throttling and is also referred to as a “deferral.”

This refusal to deliver your mail is usually temporary, but it depends on the situation. When this happens, you will get a message that says something like “user is receiving mail at too great a rate right now, please try again later,” or “user’s mailbox is over quota.” This is almost always a 400 class of error and can be cleared up within 72 hours.

Read the full article here.

We want to keep writing about topics that interest you, so keep an eye on the blog for more and check out our interactive email deliverability guide for more email terminology definitions. 

Jillian Smith
As SendGrid's Director of Content, Jillian is responsible for ensuring that SendGrid provides valuable thought leadership content through the blog, whitepapers, webcasts, and more. When not writing and editing, you can find Jillian frequenting Denver restaurants in search of the best queso.