Definitions
25 terms, each defined in one paragraph with why it matters. Jump straight to a term or read the whole list.
Bounce rate is the share of sent emails that failed to reach the recipient's inbox, reported back to the sender as either a hard or soft bounce.
Why it matters: A rising bounce rate signals list quality problems and directly damages sender reputation, which can suppress deliverability for future sends even to valid addresses.
CAN-SPAM is the United States federal law regulating commercial email, requiring accurate sender identification, non-deceptive subject lines, a physical postal address, and a working opt-out mechanism honored within 10 business days.
Why it matters: CAN-SPAM applies to any commercial email sent to U.S. recipients, and its opt-out model (rather than opt-in) is a key difference from GDPR — see /guide/compliance for the full comparison.
Click-through rate is the share of delivered emails that received at least one click on a link inside the message.
Why it matters: CTR requires a deliberate action from the recipient, which makes it a more reliable engagement signal than open rate, especially since privacy features have made opens less exact.
Conversion rate is the share of email recipients who completed the target action — a purchase, signup, or download — after receiving or clicking through an email.
Why it matters: Conversion rate ties email activity directly to a business outcome, making it the metric that matters most for ROI even though open rate and click-through rate get more day-to-day attention.
Deliverability is whether an email actually reaches the recipient's inbox, as opposed to being blocked, dropped, or filtered into a spam folder.
Why it matters: A message can be technically "delivered" (accepted by the receiving server) and still fail on deliverability by landing in spam — deliverability is the metric that determines whether anyone ever sees the email at all.
DKIM is an email authentication method that adds a cryptographic signature to outgoing messages, verified against a public key published in the sending domain's DNS records.
Why it matters: DKIM proves a message wasn't altered in transit and that it genuinely originated from an authorized server, which mailbox providers use to distinguish legitimate senders from spoofed or forged mail.
DMARC is a DNS policy record that tells receiving mail servers what to do with messages that fail SPF or DKIM checks — quarantine, reject, or take no action — and where to send reports about those failures.
Why it matters: Major mailbox providers like Gmail and Yahoo now require a DMARC policy for senders pushing meaningful volume, and messages from domains without one are increasingly rejected rather than just flagged.
Double opt-in is a signup process that requires a subscriber to confirm their email address by clicking a link in a confirmation email before they're added to the active list.
Why it matters: It filters out typos, bots, and accidental signups before they ever receive a marketing email, which keeps bounce rate and spam complaint rate lower and protects sender reputation from day one.
A drip campaign is a pre-built sequence of emails sent automatically at fixed intervals or triggered by a specific event, such as signup or purchase.
Why it matters: Drip campaigns let a single sequence built once keep nurturing every new subscriber automatically, without a marketer manually sending each message.
An ESP is the software platform used to send, manage, and track marketing or transactional email — for example, Mailchimp, Kit, or Klaviyo.
Why it matters: The ESP you choose determines your sending infrastructure, list management tools, automation depth, and reporting — see /tools for a structured comparison of 15 ESPs.
GDPR (General Data Protection Regulation) is the EU/UK law governing the collection and processing of personal data, including email addresses, and requiring clear affirmative consent before marketing email is sent.
Why it matters: GDPR applies to anyone emailing subscribers located in the EU or UK regardless of where the sending business is based, and it requires opt-in consent rather than the opt-out standard used by CAN-SPAM.
A hard bounce is a permanent delivery failure — the address doesn't exist, the domain is invalid, or the server has permanently blocked the sender.
Why it matters: Continuing to send to hard-bounced addresses signals poor list hygiene to mailbox providers, so every ESP automatically suppresses addresses after a hard bounce.
List churn is the rate at which subscribers become inactive, unsubscribe, or have their addresses go invalid over a given period.
Why it matters: Every list churns over time regardless of quality, which means sustained list growth requires new signups just to offset natural churn, not just to grow net numbers.
List hygiene is the ongoing practice of removing invalid, inactive, or unengaged addresses from an email list — including hard bounces, chronic soft bounces, and long-inactive subscribers.
Why it matters: Good list hygiene keeps bounce rate and spam complaint rate low, which protects sender reputation and keeps the rest of the list deliverable.
Open rate is the share of delivered emails that were opened by a recipient, typically tracked with a tiny invisible tracking pixel loaded when the email renders.
Why it matters: It's the most commonly cited email metric, but Apple Mail Privacy Protection now pre-fetches images for many users, which can inflate open rate independent of whether a person actually read the email — treat it as directional rather than exact.
Segmentation is the practice of dividing an email list into smaller groups based on shared characteristics — behavior, purchase history, engagement level, or demographic data — to send more relevant messages.
Why it matters: Segmented sends consistently outperform one-size-fits-all broadcasts on open rate and click-through rate because the content matches what each group actually cares about.
Sender reputation is a score mailbox providers assign to a sending domain and IP address based on historical sending behavior — bounce rate, complaint rate, engagement, and volume consistency.
Why it matters: A strong sender reputation is what gets email into the inbox instead of spam; it's built gradually over time and can be damaged quickly by a single bad send to a poor-quality list.
Single opt-in is a signup process that adds a subscriber to an active list immediately after they submit a form, with no confirmation email required.
Why it matters: Single opt-in grows a list faster than double opt-in but admits more typos, bot signups, and low-intent subscribers, which can raise bounce rate and spam complaint rate over time.
A soft bounce is a temporary delivery failure — the recipient's inbox is full, the server is temporarily unavailable, or the message was too large — that may succeed on a later attempt.
Why it matters: Repeated soft bounces on the same address over time behave like a quality signal similar to a hard bounce, and most ESPs will suppress an address after a set number of consecutive soft bounces.
Spam complaint rate is the share of recipients who marked an email as spam rather than using the unsubscribe link.
Why it matters: Mailbox providers treat spam complaints as the strongest negative signal available; Gmail and Yahoo both apply stricter sending requirements to senders whose complaint rate exceeds roughly 0.3%.
SPF is a DNS record that lists which mail servers are authorized to send email on behalf of a domain.
Why it matters: Receiving mail servers check the sending server's IP address against a domain's SPF record; a missing or misconfigured SPF record is one of the most common causes of email landing in spam.
A sunset policy is a rule for automatically removing or suppressing subscribers who haven't opened or clicked an email within a defined period, such as 6 or 12 months.
Why it matters: Continuing to email long-inactive subscribers drags down open rate and raises spam complaint risk without adding revenue, so a sunset policy protects deliverability for the rest of the list.
Transactional email is a message triggered by a specific user action — a receipt, password reset, shipping confirmation, or account notification — rather than a marketing campaign.
Why it matters: Transactional email is generally exempt from some marketing-consent rules but must still avoid promotional content mixed into the same message, and many ESPs price or send it through a separate infrastructure from marketing email.
Unsubscribe rate is the share of recipients who opted out of future emails from a given send.
Why it matters: A low, steady unsubscribe rate is normal list hygiene as uninterested subscribers self-select out; a sudden spike usually signals a content, frequency, or relevance problem worth investigating immediately.
Warm-up is the practice of gradually increasing sending volume on a new domain or IP address — starting small and to the most engaged subscribers — over roughly two to four weeks.
Why it matters: A brand-new sending domain has no reputation history, so mailbox providers apply extra scrutiny; sending at full volume immediately raises the risk of being filtered as spam before any reputation is established.