Let’s face it, email terminology can be really intimidating. With so many acronyms and interchangeable terms, it’s hard to know what’s what. To help you make sense of it all, we’ve defined some key email delivery terms for you below.


When an email recipient identifies an email message as spam or junk by clicking the “report spam” or “mark as junk” button within their email reader. A sender’s complaint rate is calculated by dividing the total number of emails received [by the ISP] by the number of complaints reported by that ISP’s customers.

Deny list:

A list of IP addresses that are known to send unsolicited and/or unwanted emails. ISPs and enterprises use deny lists to identify and filter illegitimate mail streams.


A message that is returned to the server that sent it. Bounced emails are classified as either “hard” or “soft”. A hard bounce indicates a permanent failure due to a non-existent address or a blocking condition by the receiver. A soft bounce means there has been a temporary failure due to a full mailbox or unavailable server.

Bulk mail Folder:

Also called “spam” or “junk” folder, the folder where questionable email is routed. Dedicated Ip address: An IP address or IP range that is dedicated to a specific domain and organization.


A named Internet address that resolves to the numbered Internet Protocol (IP) addresses computers use to connect.

DomainKeys Identified mail (DKIM):

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. For more info on DKIM, check out this blog post.

Domain Name System (DNS):

DNS translates a domain name into an IP address to find the owner’s site.

Email authentication:

Technical standards to help ISPs and other receivers validate the identity of an email sender. There are three authentication standards in use: Sender Policy Framework (SPF) developed by AOL, SenderID developed by Microsoft and DomainKeys Identified Mail (DKIM) developed by Yahoo!

Feedback Loop:

The process by which an ISP forwards emails reported as spam (see complaint) for immediate removal by the sender. For a deeper dive into feedback loops check out this blog post.


The documentation that accompanies the body of an email message, the header contains information on the email and the route it has taken across the Internet. Email readers display the “to” (identity of the recipient) and “from” (identity of the sender) in the inbox.

IP address:

A unique number assigned to each device connected to the Internet. An IP address can be dynamic, meaning it changes each time an email message is deployed, or it can be static meaning it does not change. A static IP address is recommended for senders of commercial email.

MTA (Mail Transfer Agent):

Software that transfers electronic mail messages from one computer to another using a client–server application architecture. An MTA implements both the client (sending) and server (receiving) portions of the SMTP (Simple Mail Transfer Protocol).

MX Record:

A Mail Exchanger (MX) record in the DNS system specifies a mail server responsible for accepting email addresses on behalf of a domain. The MX records associated with a domain assure that the email is properly routed via Simple Mail Transfer Protocol (SMTP).

Open Relay:

An SMTP server configured in such a way that it allows anyone on the Internet to send email through it, not just mail destined for or originating from known users. This is not a recommended configuration because it can be exploited by spammers – and servers with open relays are routinely blocked and/or deny listed.


Technique for acquiring information such as user names, passwords, credit cards, social security numbers and other personal data by masquerading as a trusted business like a bank or credit card company. With phish messages, the email appears to be sent by the trusted entity and the consumer is tricked into providing their personal information.

Reverse DNS:

The resolution of an IP address to a designated domain name. The reverse of the process where computer networks use DNS to determine the IP address associated with a domain name.


An email authentication standard developed by Microsoft that compares the email sender’s “From” address to the IP address to verify that it is authorized to send email from that domain.

Shared IP Address:

In the context of deploying email, this means that a single IP address or IP range is used to send email for multiple domains. The reputation of this IP is based on the aggregate performance of all the senders that use it.

SPF (Sender Policy Framework):

An email authentication standard developed by AOL that compares the email sender’s actual IP address to a list of IP addresses authorized to send mail from that domain. The IP list is published in the domain’s DNS record.

SMTP: Simple Mail Transfer Protocol

The server-to-server process used to send email across the Internet. (If you’re interested in a more detailed look at SMTP, we break it down for you in an SMTP 101 here.

Spam Filter:

Software filters that block email on a range of attributes from words or phases within the email to header information and other factors. The goal is to identify spam before it is delivered to the inbox. For more on how to stay out of the spam folder, read our free Tips and Tricks Guide.

Spam Trap:

Also called a “honeypot”, email addresses are created (or re-activated) by ISPs specifically to lure spammers. In many cases, the only way to acquire the address is through an automated email address harvesting process. Spam traps can be tricky, so check out this blog post for more tips on how to avoid them.


Technique where forged email addresses are used to trick recipients into opening an email because the source has been hidden. This deceptive tactic is used to spread viruses and other malicious programs.

Suppression List:

A list of email addresses kept by an organization that cannot be mailed because the recipients have request removal either by unsubscribing or by logging a complaint.

Unknown User:

A hard bounce error indicating the email address (user) does not exist at the organization or domain.

Allow list:

A list of trusted IP addresses and domains for which all mail is delivered, bypassing spam filters.

WHOIS Record:

A record of domain registration whereby you can discover when and by whom a domain was registered along with contact information and expiry dates.

To see how all of these terms contribute to the bigger email picture, download our free Email Infrastructure and Email Deliverability Guides.

As Twilio SendGrid's Sr. 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.