Glossary

Authentication and delivery

Term	Meaning
SPF	Sender Policy Framework. DNS record authorising which servers may send for your domain.
DKIM	DomainKeys Identified Mail. A cryptographic signature verifying the message was not altered and came from an authorised source.
DMARC	Policy telling receivers what to do on SPF or DKIM alignment failure, with aggregate reporting.
BIMI	Brand Indicators for Message Identification. Puts a verified logo into supporting clients, on top of aligned DMARC.
MPP	Apple Mail Privacy Protection. Prefetches images through Apple proxies, inflating and corrupting the open metric.
Deliverability	Whether mail reaches the inbox rather than spam. One layer below the category aware retrieval gate.
Category-aware retrieval gate	The quality-and-engagement layer above classical spam filtering that decides whether mail is surfaced in intelligent inbox views, search, and assistant answers. A sender can pass authentication and still fail it.
One-click unsubscribe	A `List-Unsubscribe-Post` header (RFC 8058) that lets a recipient opt out in a single tap from the inbox UI. Required of bulk senders alongside the in-body link.
Bounce	A failed delivery. Hard bounces are permanent and must be suppressed immediately; soft bounces are transient.
Sender reputation	The provider’s running assessment of a sending domain and IP, driven heavily by engagement.
MTA	Mail Transfer Agent. The server that routes email between hosts over SMTP; an ESP is a managed fleet of them.
SMTP	Simple Mail Transfer Protocol. The scripted command-and-response conversation MTAs use to relay mail (RFC 5321).
Envelope	The `MAIL FROM` and `RCPT TO` addresses that route a message, separate from the visible `From`/`To` headers; bounces go to the envelope sender (return-path).
DSN	Delivery Status Notification. The asynchronous bounce a receiving server mails back after it accepts a message and then fails to deliver it (RFC 3464).
Feedback loop (FBL)	A provider service that forwards spam complaints back to the sender in Abuse Reporting Format (ARF) for suppression.
Webhook	An HTTP callback a provider POSTs to an endpoint you expose, reporting an event (bounce, delivery, click, dead token) asynchronously and usually at least once.

List and data

Term	Meaning
Single opt in	Subscriber added on form submission, no confirmation step.
Double opt in	Subscriber must confirm via a link before being added. Better for quality and consent records.
Soft opt in	A narrow lawful basis (PECR) for marketing existing customers about similar products without prior express consent, provided an opt-out was offered at collection and in every message.
Sunset	Moving persistently disengaged contacts out of regular sending, after a re-engagement attempt fails, to protect deliverability for the active cohort.
List decay	The continuous loss of a list’s value as contacts change addresses, disengage, or go dormant; net list growth is acquisition minus decay.
Database health	The ongoing practice of keeping the contactable list deliverable and valuable through hygiene, re-engagement, and sunsetting.
Lead magnet	An incentive offered in exchange for contact details.
Segment	A subset of the list, dynamic (auto updating) or static (fixed at creation).
Merge tag	A placeholder replaced with subscriber specific data at send time.
Suppression list	Addresses that must never receive mail: unsubscribes, complaints, hard bounces.
Zero party data	Data a customer proactively and voluntarily shares. The cleanest signal.
First party data	Data from direct interactions with your own platforms.
Third party data	Data bought from outside aggregators. Fading as third-party cookies are phased out.

Customer data and CRM

Term	Meaning
CRM	Customer relationship management. The system of record for known customers and the interactions with them.
CDP	Customer data platform. Ingests data from every source, resolves identity, and maintains an activation-ready unified profile.
Data warehouse	Central store built for analysis and reporting at scale, not real-time activation.
Single customer view	One unified profile per person, assembled from every source.
Identity resolution	Stitching identifiers across devices and channels into one customer profile.

Segmentation and lifecycle

Term	Meaning
RFM	Recency, Frequency, Monetary. A behavioural segmentation model scoring customers on how recently, how often, and how much they buy.
CLV / LTV	Customer lifetime value. The total past and predicted value of a customer over the relationship.
Propensity model	A model predicting the probability a customer takes an action, such as buying or churning.
Churn	A customer lapsing or ending the relationship; churn rate is the share who do so in a period.
Retention rate	The share of customers retained over a period; its inverse is churn.
Lifecycle stage	Where a customer sits in the journey: acquisition, onboarding, engagement, retention, or winback.
Winback	The lifecycle stage, and the messaging it carries, that tries to reactivate a lapsed customer before suppressing the unresponsive.
Engagement score	A composite measure of how recently and often a contact engages (opens, clicks, sessions, purchases), used to trigger lifecycle stage transitions.
Engagement tier	An engagement score banded into a few groups, such as active, moderate, and dormant, used to set frequency caps and channel eligibility.
Loyalty program	A structured points, tier, or referral scheme that rewards repeat business.
Referral	Acquisition through an existing customer recommending the brand.

Channels and design

Term	Meaning
Transactional message	A message sent in response to a customer action or relationship (receipt, password reset, shipping update), governed by the contract rather than marketing consent. Distinct from a marketing message by primary purpose.
Marketing message	A message sent to drive a commercial outcome (campaign, promotion, winback), requiring marketing consent and an unsubscribe path.
Triggered (behavioural) message	A send fired by a customer’s behaviour or state, rather than a schedule.
Broadcast (batch)	A send to a segment on a schedule.
Orchestration	Deciding, per customer and moment, which message goes on which channel, and whether to send.
Frequency cap	A hard ceiling on messages per channel and in total over a rolling window.
Preheader (preview text)	The snippet shown after the subject line in the inbox; the third part of the envelope.
Responsive design	Layout that adapts to the screen it renders on; most email opens are on mobile.
Dark mode	A client display mode that recolours messages; designs must survive the inversion.
Alt text	Text shown in place of an image that is blocked or fails to load.
A2P 10DLC	The US registration standard for application-to-person SMS over long codes.
Sender ID	The label shown as the source of an SMS, either a phone number or an alphanumeric brand name. Registers such as Australia’s bind the brand label to its owner; an unregistered one is relabelled `Unverified`.
RCS	Rich Communication Services. The richer successor to SMS, with branding, cards, and read receipts.
APNs / FCM	Apple Push Notification service and Firebase Cloud Messaging, the two push delivery pipes.
Device token	The opaque, per-install identifier a push notification is addressed to; the OS can rotate or invalidate it, and dead tokens must be pruned like hard bounces.
SMPP	Short Message Peer-to-Peer. The binary protocol for submitting SMS to an aggregator or carrier, beneath the REST APIs.
DLR	Delivery receipt. The carrier’s advisory, and not always reliable, confirmation that an SMS reached the handset.
Aggregator (CPaaS)	The intermediary holding carrier interconnects that routes your SMS onto mobile networks; you integrate with it, not the carriers.

Metrics

Term	Meaning
Open rate	Share of delivered mail where the pixel fired. Corrupted by MPP; directional only.
Click through rate	Share of delivered mail that produced a link click. The workhorse metric.
Click to open rate (CTOR)	Clicks divided by opens. Content engagement, independent of open noise.
Conversion rate	Share that produced the downstream target action. The metric that pays.
Complaint rate	Share marked as spam. A hard deliverability limit; keep under 0.1%.
Reply rate	Share that replied. A strong positive inbox signal even at 1 to 2%.

Decisioning and measurement

Term	Meaning
Decision support	Machine learning that assists a journey the marketer designed.
Decisioning	Machine learning that decides per user what to send, when, and whether to send at all.
Holdout	A randomised group deliberately left unmessaged, to measure incremental effect.
Uplift	The incremental effect of sending, against an identical user left alone.
Persuadable	A user a message wins over who would not have acted otherwise.
Contextual bandit	An algorithm that balances exploring uncertain options against exploiting known good ones.
CUPED	Controlled experiment using pre experiment data. A variance reduction technique.
DiD	Difference in differences. Estimates an effect by comparing change across treated and untreated cohorts over a date.

Citations

[1] Tealium, what is a CDP [2] TechTarget, RFM analysis [3] Litmus, email preview text