A list of useful terms in terms of email deliverability. 

Acceptance Rate

The percentage of email messages that are accepted by the mail server.

Allow List

The opposite of a blocklist, this means your server is considered as an “approved sender.” It’s often used by email applications to allow users to mark whether or not they trust emails from specific senders.  This overrides some of the filtering that may exist from the ISP/receiving domain.  While not a guarantee to end up in the inbox, a sender may receive preferred delivery.

Read more here.

Blocklist

Over 500 organizations work to monitor and block web addresses that produce spam. There are different types of block lists that can occur:

• IP Blocking: When you send an email, it will originate from an IP Address. When an IP is blocked this instructs anyone who utilizes that blocklist to block the mail originating from that IP Address.

• Domain Blocking: If your Domain appears frequently in emails that hit spam traps, there may be a chance that your entire domain will be added to a blocklist. This can be even more damaging than IP Blocking as the block is not localized to just an IP Address. It will therefore affect all your sending platforms. For this reason it is helpful to use a separate sub-domain for your email marketing.

Read more about blocklists here.

Bulking

Bulking happens when an ISP/receiving email server accepts your email, but routes the message to a Bulk or Spam folder instead of routing to the inbox.

Delivery

Refers to whether a receiver’s server accepts your email, simply meaning that it has not bounced or been returned by the mail server. 

Deliverability

Also known as Inbox Placement. Refers to where that message ends up once it is accepted by the receiver's server; in other words, the inbox, spam folder, or if it is silently deleted.

Read more about email deliverability here.

DKIM (Domain Keys Identified Mail)

DKIM is a protocol used to digitally sign a message. As a result, it ensures that the sender has permission to use the domain and that the message wasn’t tampered with in transit.

Read more about DKIM here.

DMARC (Domain-Based Message Authentication, Reporting & Conformance)

DMARC prevents phishing and spoofing. It ensures that the domain used in the From field passes either DKIM or SPF, thereby assuring that the sender has permission to send from the domain in the From field visible to the recipient.

Read more about DMARC here.

Domain

Like an IP Address, domain names refer to locations of servers and devices connected to the internet. Domain names can represent a whole bunch of different IP addresses.  A domain is everything after the ‘@’ symbol in an email address.

Domain Name System (DNS)

This system maps a human-friendly domain name into a server IP address, basically like a telephone book. An MX type DNS record specifies where mail destined for a particular domain name should be sent. A TXT type DNS record is used to contextualize information about the domain for authentication purposes.

Domain Reputation 

This has become more important in recent years. Email isn’t always sent from just one IP address or provider, so using your sending domain to track reputation allows a receiver to accumulate a reputation score across the board.

Double Opt-In

If an email marketer uses a double opt-in, for a user to be added to an email list, they get sent an email with a confirmation link after they initially opt in. After the user clicks the link, they are only then added to the ongoing marketing list/database. This is a recommended way to build a healthy email marketing list.

ESPs, or Email Service Providers

ESPs provide platforms to send commercial and transactional email on your behalf.  Examples are Google, Microsoft and BT.

Email list hygiene

The practice of analysing and cleaning up your subscriber lists, removing “dead” contacts and segmenting your contacts so that they only receive relevant content.

Feedback loops

A feedback loop allows the sender to receive a report every time a recipient clicks on the “mark as spam” or “junk” button. To maintain a clean email list, you can then suppress, or prevent, unwanted messages from appearing in that particular inbox. Subscribing to feedback loops and using this data to quickly remove contacts who are no longer interested in your email helps to maintain a positive reputation.

Ham

Email that is generally desired and isn’t considered spam or unsolicited.

Hard Bounce

A permanent failure to deliver an email, usually due to a non-existent, invalid, or blocked email address. Email addresses that result in hard bounces can negatively impact deliverability and Sender Reputation and should be removed from your email lists. Removing them doesn’t necessarily mean deleting them; you can also deactivate them or add them to a suppression list. That way, you won’t re-acquire the same invalid email address and send to it again.

Read more about bounces here.

Honeypot

A planted email address designed to catch spammers.  See Spam Trap.

ISPs, or Internet Service Providers

Provide mailboxes to end users as part of their paid and free services.

Inbox Placement Rate

Once mail is accepted into an ISP's network, this figure refers to how many messages are delivered to the inbox.

Inbox Providers

Inbox Providers include ISP-provided inboxes as well as paid or free webmail accounts and email apps. Examples of this could be Gmail, Outlook.com, Yahoo.

IP Address

A number that uniquely identifies any device connected to the internet. “IP” stands for “Internet Protocol.” An IP Address helps computers find each other on the internet. Like a telephone number assigned to a person’s name in an address book.

IP Reputation:  Reputation is attributed to an IP address based on what metrics an ISP has historically seen from that IP address and how users engage with mail that originates from it.

Open Rate

The percentage of recipients who opened your email message. When someone clicks on an email, an image pixel in the email loads and is counted as an open. Details on email metrics can be found in this article.

Read Length

The length of time from when a person opens an email, until they close it. ISPs can see this information, as this is within their own network. ESPs can't see that, but measure engagement by opens.

Receiver/receiving domain

The domain your messages are going to. A domain is everything after the ‘@’ symbol in an email address.

SPF (Sender Policy Framework)

A sender policy framework allows mail services to check that incoming mail from a specific domain has, in fact, been sent from that domain. SPF protects the envelope sender address, or return path, by comparing the sending mail server’s IP address to a master list of authorized sending IP addresses as part of the DNS Record.  So, a sender policy framework is a Domain Name System (DNS) record that says an IP or domain can send email on the domain’s behalf.

Sender Score or Sender Reputation

All senders are given a sender reputation, which ultimately determines whether ISPs deliver a message to the inbox or junk folder, or block it altogether. While ISPs look at many metrics to determine sender reputation, there are four major drivers: user engagement (i.e. Opens & Clicks), user invalids (i.e. bounces), user complaints, and spam trap hits. Domain reputation ultimately bears more weight, but Internet Protocol (IP) Reputation still validates the sender in the ISP's eyes when, say, someone opens or clicks a message.

Return Path’s free reputation rating of 0-100 for every outgoing mail server IP. Mail servers will check your Sender Score before deciding what to do with your emails. A score of over 90 is good.

Single Opt-In

If an email marketer uses a single opt-in, this usually involves taking a user's form response and immediately adding that person to a live email list.

Soft Bounce

A soft bounce means that the recipient exists, but for whatever reason, they couldn’t receive your message. Soft bounces typically indicate temporary delivery issues. This could mean a user’s mailbox is full. Though this isn’t your fault, you should eventually consider them to be the same as hard bounces. It could also mean the email you sent exceeded the maximum size the subscriber’s inbox allows. In addition, Rate-Limiting or Throttling might be at play. If you send large volumes of email to the same ISP, you might start being throttled. An ISP may only allow a certain amount of connections per hour or per day, so if you exceed that, you may get blocked.

Read more about bounces here.

Spam

Not asked for, or not work-safe email. 95% of all email sent is classified as spam.

Spam Report or Spam Complaint

When someone receives your email and labels it as “spam.”  It could be the recipient felt you didn’t have permission to email them, you were emailing too frequently, or were sending irrelevant content.

Spam Traps

Spam traps are commonly used by inbox providers and blocklist providers to catch malicious senders, but quite often, legitimate senders with poor data hygiene end up on the radar as well. A spam trap looks like a real email address, but it doesn’t belong to a real person nor is it used for any kind of communication. Its only purpose is to identify spammers and senders not utilizing proper list hygiene. For example, email that hits a dead email address will return a hard bounce. If you continue to send to that dead email address, when the mail server sees continued traffic going to the dead address, it can turn the email into a spam trap, accept the email, and report the sender as a spammer.

Read more on Spam Traps here.

Standard Success Metrics

If your emails generate positive interaction and engagement they can boost your Sender Reputation. That means your standard success metrics, like opens and clicks, matter. On the other hand, if your emails are consistently deleted without being opened or they are marked as Spam, that can reduce your Sender Reputation.

Throttling

Throttling means slowing down the sending of emails.  Technically, systems could send thousands of emails a minute. However, when an ISP sees a number of bounces within a certain time-frame, they may send a throttling message that tells the sending server to slow down or stop sending for a certain period. If that message is ignored, the ISP could escalate to blocking mail from that IP address altogether.

For new IP addresses, ISPs will generally throttle the amount of mail from that IP address until they get an idea of the type, volume, and overall characteristics of the mail being sent.

Consistency and quality are the key factors, so a new IP address will need to send mail for some extended period and in a consistent manner before the ISP will allow more mail per hour or per day.

Read more about throttling here.

TINS (This Is Not Spam)

By marking something as NOT spam, your subscribers may save you from the spam filter. This requires them to go into their spam folder and manually add your address to an allow list.

Warming Up an IP/domain

The process by which you send an ever-increasing number of emails from an IP address/domain to build up on the sending IP/domain’s reputation to successfully send a greater amount of email and have it delivered/accepted.

Read more about warming up an IP/domain here.