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Home/Glossary

Email glossary

91 terms covering deliverability, lifecycle, outreach, and conversion. Each page has a one-line definition plus a longer operator-focused explanation.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

A/B test
Send two variants of an email to comparable audiences and measure which performs better.
Above the fold
Content visible without scrolling — in email, the first 300–600 pixels of vertical space.
Allowlist
A list of senders explicitly approved to deliver mail without filtering — the modern, inclusive term for whitelist.
Authentication
Mechanisms (SPF, DKIM, DMARC) that prove an email actually came from the sender it claims to be from.
Average order value
Mean revenue per transaction — drives which email tactics (discount vs. service) make economic sense.

B

BIMI
Brand Indicators for Message Identification — displays a verified brand logo next to authenticated emails in supported inboxes.
Bounce rate
The percentage of sent emails that couldn't be delivered to the recipient inbox.
Browse abandonment
Visitor viewed products but didn't add to cart — the email trigger is awareness-stage, not urgency-stage.
Buffer / pre-warming
Sending a small, gradually-increasing volume from a new IP or domain to build sender reputation before scaling.

C

CAN-SPAM
US law (2003) governing commercial email — requires accurate sender info, working unsubscribe, and no deceptive subject lines.
Cart abandonment
A shopper adds items to a cart but doesn't complete checkout — triggering an email sequence to recover the sale.
CASL
Canada's Anti-Spam Legislation — requires consent before sending most commercial electronic messages.
CCPA
California Consumer Privacy Act — gives California residents rights to access, delete, and opt out of sale of their personal data.
Click bait
Subject lines or copy that promise more than the body delivers — short-term clicks, long-term trust erosion.
Click-through rate
The percentage of email recipients who clicked at least one link in the email.
Click-to-open rate
Of the recipients who opened the email, the percentage who clicked a link.
Cohort
A group of subscribers who share a common signup window or trigger — the unit of retention measurement.
Cold email
An unsolicited email to a recipient with whom the sender has no prior relationship — typically for outreach or sales.
Complaint rate
Percentage of recipients who marked the email as spam — must stay under 0.1% to avoid deliverability damage.
Confirmed opt-in
A subscription pattern where the user actively clicks a confirmation link after submitting their email — also called double opt-in.
Conversion rate
The percentage of email recipients who completed the desired action (purchase, signup, reply, etc.).

D

Deliverability
The discipline of getting email into the recipient's inbox, not the spam folder or quarantine.
Denylist
A list of senders, IPs, or domains explicitly blocked — the modern term for blacklist.
DKIM
DomainKeys Identified Mail — cryptographic signature that proves an email genuinely came from the sender's domain.
DMARC
Domain-based authentication policy that tells receivers what to do with unauthenticated mail and reports results.
Domain warming
Gradually ramping volume on a new sending domain so ISPs can build positive reputation before high-volume sends.
Double opt-in
Synonym for confirmed opt-in — user must click a verification link before being added to the list.
Drip campaign
A pre-scheduled sequence of emails sent over time, triggered by an event like signup or purchase.

E

Email service provider
A platform (Mailchimp, Klaviyo, SendGrid, Resend, etc.) that sends, tracks, and manages email at scale.
Email service provider (ESP)
Software that sends, tracks, and manages bulk email — Mailchimp, Klaviyo, HubSpot, Resend, Postmark, SendGrid.
Engagement
Recipient interactions with email — opens, clicks, replies, forwards. ISPs use engagement as the primary deliverability signal.
Engagement window
The lookback period (typically 30-90 days) over which receivers measure subscriber engagement for placement decisions.

F

Feedback loop
An ISP program that reports back which of your recipients marked your email as spam.
Frequency capping
A rule limiting how many emails a recipient receives in a defined period — protects against fatigue and unsubscribes.
From: line / Sender name
The visible sender name and email address shown in the recipient's inbox — the second-strongest open-rate factor after subject line.

G

GDPR
EU regulation governing personal data — for email, requires lawful basis (usually opt-in) to send marketing.
Gmail Promotions tab
Gmail's algorithmic inbox category for promotional and marketing email — not spam, but not the primary inbox.
Gmail tabs
Gmail's automatic categorization of inbound mail into Primary, Promotions, Social, Updates, and Forums.

H

Hard bounce
A permanent delivery failure — the address doesn't exist, the domain is invalid, or the recipient blocked the sender.
Hashed email
An email address converted to a one-way cryptographic hash — used for cross-platform identity without storing the plaintext address.
HTML email
An email rendered with HTML and CSS — supports images, structured layout, and styled buttons. Plain text is the alternative.

I

Inbox placement
The percentage of delivered emails that land in the recipient's primary inbox rather than spam or Promotions.
IP warming
Gradually ramping volume on a new sending IP address so ISPs can build positive reputation.

J

Junk folder
Microsoft's term for spam folder — Outlook routes filtered mail there.

K

Knockout copy
Email copy that is so compelling the reader feels they must respond — the conversion-engineering ideal.

L

Lifecycle email
Behavior-triggered email sent based on where a user is in their journey — welcome, activation, retention, winback, etc.
List broker
A vendor selling lists of email addresses — almost always a deliverability disaster.
List hygiene
The practice of keeping an email list clean — removing bounces, suppressing unengaged subscribers, validating addresses.
List-Unsubscribe header
An RFC-defined email header that lets the recipient's client offer a one-click unsubscribe button — required by Gmail and Yahoo for bulk senders.

M

Magic link
An email-delivered link that signs the user in without requiring a password.
Mail Privacy Protection
Apple's feature (since iOS 15, 2021) that preemptively loads email images — inflating open rates for Apple Mail users.
Mailbox provider
The company providing the recipient's email service — Gmail, Outlook, Yahoo, Apple, etc. Each has its own filtering and reputation system.
MIME
Multipurpose Internet Mail Extensions — the format that lets email carry attachments, HTML, and non-ASCII characters.
MX record
DNS entry that tells the internet which servers accept mail for a domain.

N

Newsletter
A recurring broadcast email on a calendar schedule — different from triggered lifecycle email.

O

Open rate
The percentage of email recipients who opened the email — historically the headline engagement metric, now corrupted by Apple Mail Privacy Protection.
Opt-in
The user actively agrees to receive email, typically by submitting a signup form or checking a consent box.
Opt-out
The pattern where the subscriber is assumed to consent until they actively decline — legal in the US, illegal in most of Europe.

P

Personalization
Tailoring email content to the individual subscriber — beyond {{firstName}}, the meaningful kind uses behavior and segment data.
Plain text email
An email with no HTML formatting — just text, no images, no styled buttons.
Preheader
The short text snippet shown in the inbox after the subject line, before the user opens the email.
Promotions tab
Gmail's tab for marketing email — separate from the Primary inbox. Engagement is lower but not zero.

Q

Quiet hours
A configured time window during which email sends are suppressed to respect recipient timezone preferences.
Quoted-printable
An email content-encoding for non-ASCII characters — usually invisible, but matters when it leaks into the body.

R

Re-engagement
Email program targeting subscribers who've gone dormant — the gentler sibling of winback.
Re-engagement campaign
A planned attempt to revive inactive subscribers before pruning them from the list.
Reply rate
The percentage of email recipients who replied — the headline metric for cold outreach and high-touch B2B email.

S

Segmentation
Splitting your list into smaller groups based on behavior, attributes, or intent — the prerequisite for relevant email.
Sender reputation
The trust score mailbox providers assign to your domain and IP, driving inbox placement.
Single opt-in
The subscriber is added to the list on form submit, without a confirmation-email step.
Soft bounce
A temporary delivery failure — mailbox full, server temporarily down, etc. Can be retried.
Spam folder
Where messages flagged as junk land — the worst inbox placement outcome short of outright rejection.
Spam trap
An email address used by ISPs and anti-spam services to catch senders with poor list hygiene.
SPF
Sender Policy Framework — DNS record declaring which servers are authorized to send mail for your domain.
Statistical significance
The threshold at which an A/B test result is unlikely to be due to chance — typically 95% confidence.
Subject line
The visible text shown in the inbox before opening — the primary lever on open rate.
Suppression list
A list of addresses that must never receive mail again — unsubscribes, hard bounces, spam complainants.

T

Throttling
ISPs slowing down acceptance of mail from a sender — often the first deliverability warning before harder filtering.
Transactional email
An email sent in response to a specific user action — receipts, password resets, shipping confirmations.

U

Unsubscribe
The mechanism by which a recipient opts out of receiving future email — legally required to be clear and prompt.
Unsubscribe rate
Percentage of recipients who unsubscribed from a given send — a lagging indicator of list fatigue or audience mismatch.
UTM parameters
URL query parameters that track which email/campaign drove traffic — utm_source, utm_medium, utm_campaign, etc.

V

Variable content
Personalized fields merged into an email body from a database or CSV at send time.
VMC
Verified Mark Certificate — the certificate required to display a brand logo via BIMI.

W

Warmup
Either the domain/IP-reputation building process, or the practice of sending fake conversations to improve cold-email deliverability.
Webhook
An HTTP callback that fires when an email event happens (delivered, opened, clicked, bounced) — for real-time data sync.
Welcome series
A 3-5 email automated sequence sent to new subscribers in the first 7-14 days after signup.
Winback
Email program targeting customers who have churned or lapsed — focused on recovery, not relevance.

X

X-Mailer
Optional email header identifying the sending software — visible to recipients, sometimes used as a spam signal.

Y

Yield (email)
Revenue or conversions produced per email sent — the unifying business metric across all email programs.

Z

Zero-party data
Data a customer intentionally and proactively shares with a brand — preferences, intentions, profile attributes.