Email and SMS marketing has accumulated its own dense vocabulary over two decades, and half the confusion clients bring to an agency conversation comes from not knowing what the terms actually mean. This glossary breaks down the terminology by category — deliverability, compliance, automation, and list mechanics — so you can walk into a strategy conversation speaking the same language as the platform dashboards and the specialists running your program.
Deliverability and Inbox Placement Terms
Deliverability is the invisible layer that determines whether a message even reaches the inbox, and it has a vocabulary all its own.
- Sender reputation — a score, tracked by mailbox providers like Gmail and Outlook, reflecting how trustworthy a sending domain or IP address is based on engagement, complaint rates, and bounce history. Low reputation means more messages land in spam.
- SPF (Sender Policy Framework) — a DNS record listing which mail servers are authorized to send email on behalf of a domain, helping receiving servers detect spoofed messages.
- DKIM (DomainKeys Identified Mail) — a cryptographic signature added to outgoing email that lets receiving servers verify the message was not altered in transit and genuinely came from the claimed domain.
- DMARC (Domain-based Message Authentication, Reporting & Conformance) — a policy layer built on SPF and DKIM that tells receiving mail servers what to do with messages that fail authentication, and reports back to the domain owner.
- Inbox placement rate — the percentage of sent messages that actually land in the primary inbox, as opposed to spam or a promotions tab. This differs from delivery rate, which only confirms the message was accepted by the receiving server.
- Hard bounce — a permanent delivery failure, typically because the address does not exist. These should be suppressed immediately.
- Soft bounce — a temporary delivery failure, such as a full inbox or a server timeout, that may resolve on a later send attempt.
Compliance and Consent Terms
Compliance vocabulary carries real legal weight, and misunderstanding these terms is where businesses run into regulatory exposure.
- CAN-SPAM Act — the U.S. federal law governing commercial email, requiring accurate sender information, a working unsubscribe mechanism, and prompt honoring of opt-out requests.
- CASL (Canada’s Anti-Spam Legislation) — Canada’s stricter consent law requiring explicit opt-in for most commercial electronic messages, with meaningful penalties for violations.
- TCPA (Telephone Consumer Protection Act) — the U.S. law governing SMS and telemarketing, requiring prior express consent before sending marketing texts, with statutory damages available per violation.
- 10DLC (10-Digit Long Code) — the registration system U.S. carriers require for businesses sending SMS from standard local-format numbers, intended to reduce spam and improve deliverability for registered senders.
- Double opt-in — a signup process requiring a subscriber to confirm their email address via a confirmation link before being added to an active list, producing a cleaner, more engaged list than single opt-in.
- Single opt-in — a signup process that adds a subscriber immediately upon form submission, without a confirmation step. Faster growth, but a higher risk of invalid or mistyped addresses on the list.
List and Segmentation Terms
- Segment — a subset of a list grouped by shared traits or behavior, such as purchase history, engagement level, or location, used to send more relevant messages than a single blast to everyone.
- Suppression list — a list of addresses or phone numbers explicitly excluded from sends, typically because they unsubscribed, bounced, or requested removal.
- List hygiene — the ongoing practice of removing invalid, bounced, or chronically unengaged contacts to protect sender reputation.
- Sunset policy — a defined rule for when to stop emailing contacts who have not engaged after a set period, often paired with a re-engagement sequence before final removal.
- RFM segmentation — grouping contacts by Recency, Frequency, and Monetary value of past purchases, a common e-commerce segmentation model.
Automation and Lifecycle Terms
- Drip campaign — a pre-scheduled series of messages sent at fixed intervals after a trigger event, such as a signup, regardless of recipient behavior in between.
- Trigger-based automation — a message or sequence launched by a specific subscriber action, like an abandoned cart, a completed purchase, or a form submission, rather than a fixed calendar date.
- Welcome sequence — the automated series of messages a new subscriber receives immediately after opting in, typically introducing the brand and setting expectations.
- Win-back campaign — a targeted sequence aimed at re-engaging subscribers who have gone dormant, often including a special offer or a direct question about continued interest.
- Lifecycle marketing — the broader strategy of tailoring messaging to where a customer sits in their relationship with the business, from first-time lead to repeat customer to lapsed customer.
SMS-Specific Terms
- A2P messaging (Application-to-Person) — business-to-consumer text messaging sent from software platforms, distinct from person-to-person texting, and subject to carrier registration requirements.
- MMS (Multimedia Messaging Service) — a text message that includes an image, GIF, or other media, as opposed to plain-text SMS.
- Short code — a short, five-or-six-digit number used for high-volume SMS sending, typically leased by large brands; distinct from 10DLC long codes used by most local and mid-size businesses.
- Two-way SMS — a texting setup that allows recipients to reply and have a real conversation with the business, as opposed to a one-way broadcast number.
- Keyword opt-in — a consent mechanism where a subscriber texts a specific word (like “JOIN”) to a business number to opt into future messages.
Measurement Terms Worth Knowing
- Open rate — the percentage of delivered emails that were opened, though this metric has become less reliable since Apple Mail Privacy Protection began pre-fetching images and inflating opens automatically.
- Click-through rate (CTR) — the percentage of delivered messages that received at least one click.
- Click-to-open rate (CTOR) — the percentage of people who opened the message and then also clicked, a cleaner measure of content relevance than raw CTR.
- Conversion rate — the percentage of recipients who completed the desired action, such as a purchase or booking, after clicking through.
- Unsubscribe rate — the percentage of recipients who opted out following a given send, an important early warning signal for content fatigue or frequency problems.
Why the Terminology Matters Beyond Vocabulary
None of these terms exist in isolation — they describe interlocking systems. A poor sunset policy inflates your sent volume to unengaged contacts, which damages sender reputation, which lowers inbox placement rate, which then makes even your best campaigns underperform. Understanding the vocabulary is really understanding the mechanics, and the mechanics are what separates a program that compounds in value from one that quietly erodes.
Frequently Asked Questions
What is the difference between deliverability and delivery rate?
Delivery rate simply measures whether a receiving server accepted the message. Deliverability is the broader concept of whether the message actually reached the primary inbox rather than spam or a secondary tab — a message can have a 99% delivery rate and still have poor real-world inbox placement.
Is single opt-in ever an acceptable choice, or should businesses always use double opt-in?
Single opt-in can be reasonable for lower-risk list building, like a point-of-sale email capture with an in-person relationship already established. Double opt-in is generally safer for any list built through paid ads or third-party forms, since it filters out mistyped or fake addresses before they damage sender reputation.
What does "engaged segment" actually mean in practice?
It refers to a defined subset of the list, usually contacts who have opened or clicked within a recent window like 90 days, that a business prioritizes for its main sends while managing less-engaged contacts on a separate, lighter cadence.
Why do businesses need 10DLC registration if they already have a phone number?
Carriers require application-to-person registration to distinguish legitimate business texting platforms from spam operations. Sending unregistered bulk SMS through a standard number risks throttling, filtering, or outright blocking by carriers.
Does Apple Mail Privacy Protection mean open rate is a useless metric now?
Not useless, but less trustworthy on its own. It is best used as a directional signal alongside click-through rate and conversion rate rather than as the primary success metric, since automatic image pre-fetching can inflate opens regardless of genuine recipient engagement.
Terry has 30+ years in software and SEO. He’s the founder of Salterra Digital Services and SEO Spring Training, host of the Roundtable SEO Mastermind, and lead instructor at SEO University — teaching the exact tactics his team uses on client work.
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