The Act-On Email Deliverability Glossary, Part 2


Glossary PART2This glossary defines the most common terms that the email marketer should be familiar with, in order to manage the many components of deliverability.

We ran Part 1 of the Deliverability Glossary last week; you can catch up with it here.



Permission-Based In a “permission-based” email marketing, emails are sent only to recipients who have given consent to be contacted.
Phishing This is an email fraud strategy in which the perpetrator sends legitimate-looking email in an attempt to gather personal identity and/or financial information from recipients. Often the messages appear to come from well-known institutions such as banks, insurance companies, or government agencies.
Placement Where an email ends up after being delivered to the intended recipient’s domain. For example, when mailing to a Gmail subscriber, your email could be placed in the Primary, Social, Promotions, Updates, or Junk folders.
Postmaster The administrator of an email server. It can be a person or an automated response. According to the Internet Engineering Task Force (IETF), any system that includes an SMTP server supporting mail relaying or delivery must support the reserved mailbox “postmaster” as a case-insensitive local name.
Queues When email is held up at a domain before being accepted or sent to the inbox, it may be kept in a queue for a time as it goes through spam appliances and filters.
Ramp-Up A specific time period in which email volume from a dedicated IP is gradually increased. This helps ensure that the IP is properly configured for successful delivery, and helps build a good sending reputation. Also known as “warming” an IP address.
Re-Engagement The strategy of trying to contact inactive customers and reestablish an active relationship. It often takes the form of asking whether the recipient wants to remain in contact, and/or making a highly targeted offer.
Remediation A process in which a representative of a domain or sending IP that has been blocked or placed on a blacklist takes steps with a receiving ISP or domain to remove the blocks or listings.
Reputation An overall picture of how trustworthy an email sender is, based on factors such as complaint rate, number of recipients, bounce rate, spam traps, sending history, etc. It’s analogous to a credit score: the lower your email sending reputation, the more scrutiny your emails will receive.
Return Path The email address for return mail.
RFC Compliant The Internet Engineering Task Force ( issues Requests for Comment (RFCs) that function as written standards. Marketers and email service providers should be compliant with RFC standards for sending email.
Safe Sender List Email recipients (in most email clients) have the option to mark senders as Safe Senders, a practice also known as whitelisting. If your sending IP address is on someone’s Safe Senders list, your emails will usually reach him or her unimpeded.
Segmentation This is the practice of segmenting your marketing lists into different groups with diverging characteristics. Examples include buyers of product A/buyers of product B; engaged/unengaged; Houston/Dallas; CEOs/managers; and so on. Segmenting allows more targeted messaging, which in turn boosts engagement, which in turn boosts deliverability.
Sender Score One of many criteria used to evaluate the trustworthiness of a sending IP. Think of your Sender Score as a credit score. Many factors can affect it. If your Sender Score is high, receiving servers are less likely to apply filtering criteria to your entire IP address (but will instead focus on filtering each email campaign and individual subscriber engagement). A Sender Score goes from 0-100, with 100 being the best.
Sending Domain The domain from which specific email correspondence originates from, and which your end recipients see when viewing the message in their email client.
Sent Number The amount of outbound “Sent” emails meant for recipients.
Shared IP An email sending environment in which IP ranges and reputations are shared among multiple senders. (See “Dedicated IP” for another option.)
SMTP Simple Mail Transfer Protocol (SMTP) is an Internet standard for email transmission. Many proprietary systems (such as Microsoft Exchange) and webmail systems (such as Gmail) use their own non-standard protocols to access mail box accounts on their own mail servers; all use SMTP when sending or receiving email from outside their own systems.
Soft Bounce A type of bounce that indicates a temporary technical problem with delivering the email (such as a connection failure). As opposed to a “Hard Bounce”, which indicates a permanent technical problem with the recipient’s address. Soft bounces can be caused by the recipient’s email inbox being full, or by an email message that is too large. Most email service providers will try for some period of time (three days is common) to deliver the email. Usually after some number of soft bounces, the address is converted to a hard bounce.
Spam Unwanted email communication of any kind, aka Unsolicited Commercial Email (UCE) or Unsolicited Bulk Email (UBE).
Spam Complaints In most email clients, people can click a “This is Junk” button of some kind to indicate that a received email message is spam. These spam complaints are noted by Internet service providers, and can be detrimental to your sending reputation and deliverability. You will see spam complaints only from global receivers and or ISPs who have Feedback Loops.
Spam Traps Invalid email addresses used to snare spammers. There are two types of spam traps, pristine and recycled. Both will hurt your sending reputation; the pristine trap will do the most damage.

  • A pristine spam trap is an email address that’s been created by an ISP or by a blacklist. If a marketer sends to a pristine trap, your IP reputation can drop and your emails can be blocked.
  • Recycled spam traps are formerly valid email addresses that have been abandoned, which an ISP repurposes as spam traps (to catch the spam-like behavior of sending to unengaged recipients). Because email addresses change frequently, lists that go uncleaned for any length of time may well contain spam traps.
Spam Triggers Any characteristics of spam emails (certain special characters, image-only emails, etc.) that marketers should avoid using in legitimate commercial emails, in order not to be profiled as a spammer.
SPF (Sender Policy Framework) Another common form of authentication designed to catch spoofing and other spam-like behavior. It checks that incoming mail from a domain comes from a host authorized by that domain’s administrators.
Spoofing A tactic a malicious sender can use to falsify a sending address to appear trustworthy and gather sensitive data. Very similar to phishing.
Text-to-Image Ratio Proportionately, how much of each is in an email message. The best practice recommendation is for text and images to be split evenly, or 60/40 text/images. Spammers often send emails that consist only of images; or have no images, very little text, and links. ISPs like to see a good balance of text to images. We recommend testing various percentages to find the right balance for your audience and your purposes.
Throttling A practice ISPs and domains use to limit the amount of incoming mail to their servers from a particular sender at any one time. One possible reason (among others): If some recipients at this domain or receiving server have marked your mail as spam, the server may refuse to receive more until it sees whether (and how) more recipients open and engage with your email.
Timestamp An email timestamp notes the time an email was sent to the outgoing mail server. It is usually within a split second of when the email reaches a receiving server and (barring blocks, etc.) is delivered to the inbox.
Unengaged Email recipients who have never opened or clicked through one of your emails, or who have not interacted in a time period you specify. 180 days is a common metric.
Unique Clicks The number of recipients who clicked on an email. No matter how many times someone clicks, they are counted only once.
Unique Opens The number of recipients who opened an email. No matter how many times they open it, they are counted only once.
Unsubscribe By law, every commercial email must offer the recipient the choice to no longer receive email messages or other communications. This is most often seen as an “Unsubscribe” link.
Volume The number of total recipients that a sender mails to during a defined time period, such as a day or month.
Volume & Frequency Management It’s a best practice to keep the volume and frequency of mailings consistent and even. Sudden spikes of either can lead to deliverability issues.
Whitelists ISPs and corporate domains may keep whitelists: Lists of domains and IP addresses that are always considered safe. Email from a whitelisted sender will not be deleted or sent to the junk mail folder by the spam filter. End users can add to the whitelist when they receive an email and mark the sender as a Safe Sender. They can also cause the sender to be de-whitelisted if enough of them mark the sender’s messages as spam.

Did you find this useful? Is there anything missing? Let us know in the Comment box, please.

DeliverabilityThe art of successful email marketing depends first and last upon proactive deliverability management. Download the white paper, Best Practices in Email Deliverability, and learn the best practices for how to manage the critical factors that affect the deliverability of your email messages.

You will learn about what email service providers look for, SPAM facts, the key factors that affect your sender reputation, best practices for retention and acquisition, and so much more!