A
A/B test
An A/B test (also: split test) is an experiment where you compare two versions of an email. Half of your list receives version A, the other half version B. By measuring which variant performs better — higher open rate, more clicks or conversions — you optimize based on data. Test subject lines, sender names, send times or CTA buttons. Many ESPs automatically send the winning version to the rest of your list.
Above the fold
Above the fold is the part of your email that is visible without scrolling (roughly 300-500 pixels). Your core message and primary CTA should be placed here. Research shows that recipients decide within 3 seconds whether to keep reading.
AMP for Email
AMP for Email is a Google technology that lets you embed interactive elements in emails: carousels, forms, accordions and real-time content. Supported by Gmail and Yahoo Mail. Offers opportunities for surveys and live inventory displays, but requires additional technical implementation.
Authentication
Email authentication is the process by which mail servers verify that an email actually comes from the stated sender. The three protocols: SPF, DKIM and DMARC. Since 2024, Google and Yahoo require bulk senders to have all three correctly configured.
Automation
Email automation is the automatic sending of emails based on triggers and rules. Think of a welcome series after signup, abandoned cart reminders or a nurture sequence. With AI, you can make automations even smarter by personalizing send times and content based on behavioral data.
Autoresponder
An autoresponder is an automatic email sent immediately after a specific action, such as a form submission or an order. The simplest form of automation — a single, pre-written response. Modern autoresponders are often part of broader workflows.
B
Behavioral email
A behavioral email is automatically triggered by a recipient's behavior: viewed a product but didn't buy, downloaded an ebook, or went inactive. These emails achieve 3-5x higher click rates than regular newsletters because they respond to concrete actions.
Blacklist
A blacklist (blocklist) is a list of IP addresses or domains flagged as spam senders. Well-known blacklists: Spamhaus, Barracuda and SORBS. You end up on one due to high bounce rates, spam complaints or sending to invalid addresses. Check your status via MXToolbox and maintain good list hygiene.
Bounce
A bounce is an email that cannot be delivered. Two types: hard bounce (permanent, e.g. nonexistent address) and soft bounce (temporary, e.g. full inbox). Keep your bounce rate below 2% and remove hard bounces immediately.
Bounce rate
The bounce rate is the percentage of undelivered emails: (bounces / sent) x 100. Healthy: below 2%. Above 5%: action needed, you risk blacklisting. Regular list hygiene is the solution.
Bulk email
Bulk email is simultaneously sending an email to a large group of recipients who have given consent (opt-in). Think newsletters and seasonal campaigns. Segmenting and personalizing is essential to avoid being flagged as spam.
C
CAN-SPAM
The CAN-SPAM Act is the US law governing commercial email. Key requirements: no deceptive subject lines, include a physical mailing address, provide a working unsubscribe option. In Europe, the stricter GDPR requires prior consent (opt-in). Sending internationally? Follow the strictest applicable law.
Click rate (CTR)
The click rate (click-through rate) is the percentage of recipients who click a link in your email: (unique clicks / delivered emails) x 100. Average is 2-3%, but varies by industry. Improve with relevant content, clear CTAs and good segmentation.
Click-to-open rate (CTOR)
The CTOR measures the percentage of openers who also click: (unique clicks / unique opens) x 100. A purer measure of content effectiveness than CTR, since it excludes the influence of your subject line. Benchmark: 10-15%.
Confirmation email
A confirmation email confirms an action: double opt-in confirmation or transaction confirmation. These emails have the highest open rates — often above 70%. Leverage that attention with a clear next step.
Conversion rate
The conversion rate in email marketing is the percentage of recipients who complete the desired action (purchase, signup, download): (conversions / delivered emails) x 100. The ultimate measure of campaign effectiveness.
CTA (Call-to-Action)
A CTA in email is the button or link directing toward the desired action. Effective CTAs are: visually prominent (button), concise (2-5 words), action-oriented ("Download Now") and above the fold. Use a single CTA per email when possible.
D
Dedicated IP
A dedicated IP is a unique IP address exclusively for your email sending. You build your own sender reputation, independent of other senders. Downside: you need a minimum of 25,000-50,000 emails per month for a stable reputation. For smaller senders, a shared IP is often better.
Deliverability
Deliverability is the ability to get emails into the inbox, not into spam. Determining factors: sender reputation, authentication (SPF, DKIM, DMARC), list hygiene and engagement. A good rate is above 95%. Monitor via Google Postmaster Tools.
DKIM
DKIM (DomainKeys Identified Mail) adds a digital signature to your emails. The receiving server uses this to verify that the email came from your domain and was not altered. It works with a public key in your DNS and a private key on your mail server. One of the three pillars alongside SPF and DMARC.
DMARC
DMARC (Domain-based Message Authentication, Reporting & Conformance) builds on SPF and DKIM. It tells mail servers what to do with emails that fail authentication: nothing (none), quarantine or reject. Start with "none" to gather data, then work toward "reject" for maximum protection.
Double opt-in
Double opt-in is a two-step signup process: after form submission, the person receives a confirmation email with a click link. This ensures the address is valid and the owner genuinely wants to receive emails. Result: cleaner list, lower bounce rates, higher engagement. In the EU, double opt-in is the recommended method under GDPR.
Drip campaign
A drip campaign is an automated email sequence sent at fixed intervals after a trigger (day 1, 3, 7, 14). Recipients are guided step by step toward a conversion. Typical forms: welcome series, onboarding flows and lead nurture sequences. Set up once, runs automatically for every new recipient.
Dynamic content
Dynamic content is email content that automatically changes per recipient: product recommendations based on purchase behavior, location-specific offers or segment-dependent blocks. One campaign, but different for every recipient. Platforms like HubSpot, ActiveCampaign and Mailchimp support this.
E
Email client
An email client is the software recipients use to read email: Apple Mail (~50%), Gmail (~30%), Outlook (~8%). Each client renders HTML differently, so always test across multiple clients. Tools like Litmus and Email on Acid automate this.
Email list
An email list (mailing list) is your collection of email addresses from people who have given consent. Quality matters more than size: 5,000 engaged subscribers generate more results than 50,000 inactive addresses. Build organically via opt-in forms. Never buy lists — it damages your sender reputation and violates GDPR.
Engagement
Engagement measures how actively recipients interact: opens, clicks, replies, forwards. High engagement improves your deliverability, low engagement worsens it. Monitor per segment and re-engage or remove long-term inactive subscribers.
ESP (Email Service Provider)
An ESP is the platform for email campaigns: Mailchimp, ActiveCampaign, HubSpot, Klaviyo, Brevo, MailerLite. It offers an editor, automation, segmentation, A/B testing and analytics. Increasingly, ESPs are integrating AI for subject line suggestions and send time optimization.
F
Feedback loop (FBL)
A feedback loop is a service from mail providers that informs your ESP when someone marks your email as spam. This lets you automatically unsubscribe that recipient. Keep spam complaints below 0.1% — above that, it seriously damages your sender reputation.
From name
The from name is the sender name displayed in the inbox. One of the most important factors for open rate: people open emails from recognizable senders. "Ruud from Searchlab" often works better than just "Searchlab" — a personal name is more recognizable.
G
GDPR
The GDPR (General Data Protection Regulation) is the European privacy law (since 2018). For email marketing: explicit opt-in is required, easy unsubscribe must be provided and consent must be provable. Fines up to 4% of annual revenue.
Graymail
Graymail is email that was once consented to, but is no longer being read. It lowers your engagement and gets increasingly routed to spam. Solution: re-engagement campaigns and removing inactive subscribers.
H
Hard bounce
A hard bounce is a permanently undeliverable email: the address doesn't exist, the domain is invalid or the server blocks you structurally. Remove immediately from your list — resending damages your sender reputation. Keep below 0.5%.
HTML email
An HTML email contains visual formatting: images, colors, buttons, brand styling. The opposite of plain text. Use inline CSS, test across multiple clients (Outlook doesn't support flexbox/grid) and always provide a plain text fallback.
Honeypot
A honeypot is a hidden email address set up by anti-spam organizations to identify spam senders. Sending to a honeypot = blacklist. This happens with purchased lists or poor list hygiene. Yet another reason to never buy lists.
I
Inbox placement
Inbox placement is the percentage of emails that land in the primary inbox (not spam or promotions). Different from deliverability: an email can be delivered but still end up in spam. Measure via GlockApps or Everest.
IP warming
IP warming is the gradual increase of volume on a new dedicated IP. Start with 500/day, increase over 4-8 weeks. During the warming period, send to your most active subscribers — their engagement builds a positive reputation.
K
KPIs (email marketing)
Key KPIs: open rate, click rate, CTOR, conversion rate, bounce rate, unsubscribe rate and spam complaint rate. Which matters most depends on your goal. Track trends over time, not individual campaigns.
L
Landing page
A landing page is the page recipients reach after clicking a link in your email. The alignment between email and landing page determines your conversion. Make sure the promise is fulfilled, use consistent design and make the desired action simple.
Lead magnet
A lead magnet is something of value offered in exchange for an email address: ebook, checklist, template, free tool or discount. The best lead magnets solve a specific problem and align with what you ultimately sell.
Lifecycle email
A lifecycle email is tailored to the stage of the customer journey: welcome series, lead nurturing, onboarding, upsell/cross-sell, retention and win-back. The right message at the right time, aligned with where someone stands in the relationship with your brand.
List hygiene
List hygiene is regularly cleaning your email list: removing hard bounces, inactive subscribers, duplicates and invalid addresses. Improves deliverability, reduces costs and provides a clearer picture of engagement. Perform at least quarterly.
List segmentation
List segmentation is dividing your list into groups based on demographics, behavior, interests or customer journey stage. Segmented campaigns average 14% higher open rates and 100% more clicks. Start with active vs. inactive and refine from there.
List-Unsubscribe header
The List-Unsubscribe header enables mail clients to display an "Unsubscribe" link next to your sender name. Required for bulk senders since 2024. Better that someone unsubscribes than marks you as spam — the latter damages your reputation far more.
M
Mail merge
Mail merge combines a template with recipient data for personalized emails. The foundation of personalization — from name insertion to complex dynamic content.
Marketing automation
Marketing automation goes broader than email automation: automation across multiple channels (email, social, ads, website). Encompasses advanced workflows with lead scoring and multi-channel triggers. Research shows that businesses automate an average of 3+ channels.
Multivariate test
A multivariate test tests multiple variables simultaneously (e.g. subject line x preheader x image). Reveals which combination performs best. Downside: you need significant volume (50,000+ subscribers) for statistically significant results.
N
Newsletter
A newsletter is a periodic email with news, tips or content. Effective newsletters: provide value (not just promotions), maintain a consistent frequency and use a recognizable format. Builds trust and keeps your brand top-of-mind.
Nurture sequence
A nurture sequence is an automated series that warms up leads toward conversion: first education, then social proof (case studies, reviews), and finally an offer. Typical B2B: 4-8 weeks, 5-10 emails. The content ideally adapts based on the recipient's behavior.
O
One-to-one email
A one-to-one email is a personal message to an individual, as opposed to mass mailing. Sales tools like HubSpot Sales and Instantly make it possible to send one-to-one emails at scale that look like personal messages.
Open rate
The open rate is the percentage of recipients who open your email: (unique opens / delivered emails) x 100. Average is 20-25%. Note: Apple Mail Privacy Protection automatically registers opens, which artificially inflates numbers. Use in combination with click rate and CTOR.
Opt-in
Opt-in is explicit consent to receive emails. Required under GDPR. Two forms: single opt-in (direct) and double opt-in (additional confirmation email). Must be specific, freely given and informed — pre-checked boxes don't count.
Opt-out
Opt-out is the unsubscribe process via an unsubscribe link or List-Unsubscribe header. Legally required in every commercial email. Make it easy — the harder it is, the more spam complaints you get.
P
Permission marketing
Permission marketing (a term coined by Seth Godin) is the philosophy of only emailing people who have given consent (opt-in). Unsolicited emailing is not only ineffective but also illegal in Europe under GDPR.
Personalization
Personalization goes beyond first names: product recommendations, industry-specific content, optimal send times and dynamic content per segment. Personalized emails generate 6x higher transaction rates. AI makes personalization at scale increasingly accessible.
Plain text email
A plain text email contains only text, no formatting. Advantages: renders correctly in all clients, passes spam filters more easily and feels more personal. In B2B, plain text emails often outperform HTML designs. Always include a plain text version as a fallback.
Preheader
The preheader (preview text) is the short text next to or below the subject line in the inbox. Your second chance to earn an open. Best practices: complement the subject line (don't repeat it), 40-130 characters, be specific. Without a preheader, the client displays the first lines of your email — often an unfortunate result.
Preference center
A preference center lets subscribers manage their preferences: type of content, frequency and areas of interest. Reduces your unsubscribe rate because it offers an alternative to unsubscribing entirely.
R
Re-engagement campaign
A re-engagement campaign targets inactive subscribers. Tactics: "Do you still want to hear from us?", a special offer, or showing what they're missing. No response after 2-3 attempts? Remove them for better list hygiene.
Reply rate
The reply rate is the percentage of recipients who reply. Especially relevant for sales outreach. Cold: 1-5%, warm: 10-20%. Also a positive signal for mail providers — it proves recipients find your emails valuable.
Responsive email
A responsive email adapts to the screen size (desktop, tablet, mobile). With 60%+ of emails opened on mobile, it's a requirement. Techniques: media queries, scalable images and buttons of at least 44x44 pixels.
S
Seed list
A seed list is a set of test addresses (Gmail, Outlook, Yahoo) to check whether your emails are correctly delivered and displayed before you send the full campaign. Proactive deliverability monitoring.
Sender reputation
Sender reputation is the score mail providers assign to your domain and IP, based on bounces, spam complaints, engagement and consistency. Good reputation = inbox. Bad reputation = spam. Check via Google Postmaster Tools. Slow to build, quick to lose.
Sender score
Sender Score is a reputation score from Validity on a scale of 0-100, based on 30 days of sending behavior. Above 80 is good, above 90 is excellent, below 70 means trouble. Free to check at senderscore.org.
Single opt-in
Single opt-in: direct addition to your list after form submission, without a confirmation email. Lower barrier but higher chance of invalid addresses. Double opt-in offers stronger legal protection under GDPR.
Soft bounce
A soft bounce is a temporary delivery issue: full inbox, server offline or message too large. ESPs automatically retry (24-72 hours). After 3-5 repeated soft bounces, the address is treated as a hard bounce.
Spam
Spam is unwanted, unsolicited bulk email — the opposite of permission marketing. Spam filters evaluate reputation, content and recipient behavior. Sending spam is illegal under GDPR and CAN-SPAM.
Spam complaint
A spam complaint occurs when someone clicks "This is spam." The most heavily weighted negative signal for your reputation. Keep below 0.1% (1 per 1,000). Above 0.3% you risk being blocked.
Spam filter
A spam filter scans incoming emails for authentication, reputation, content and sending patterns. Avoid triggers: "FREE" in all caps, too many exclamation marks, deceptive subject lines and poor image-to-text ratio.
Spam trap
A spam trap identifies spam senders. Two types: pristine traps (never used, like honeypots) and recycled traps (old addresses repurposed as traps). Solution: double opt-in, never buy lists, regular list hygiene.
SPF
SPF (Sender Policy Framework) specifies in your DNS which mail servers are allowed to send on behalf of your domain. Server not in the SPF record? The email may be rejected. One of the three pillars alongside DKIM and DMARC.
Subject line
The subject line is the most important factor for your open rate, along with the from name. Best practices: 30-50 characters, specific, no deception, no ALL CAPS. Test via A/B tests.
Subscriber
A subscriber is someone who has actively given opt-in consent. Treat them as a valuable community: provide consistent value, respect preferences and make unsubscribing easy. Quality outweighs quantity.
Suppression list
A suppression list contains addresses you never email again: unsubscribed subscribers, hard bounces and spam complaints. You intentionally retain them to prevent them from being re-added via other sources. Legally required.
T
Template
An email template is a reusable email layout. Good template: responsive, brand-consistent, modular and tested in the major clients. Custom templates that match your brand identity outperform standard ESP templates.
Throttling
Throttling is slowing down your send speed: spread 100,000 emails over hours or days. Prevents mail providers from flagging you as suspicious. Especially important for new domains and IP warming.
Transactional email
A transactional email is triggered by a user action: order confirmations, password resets, invoices. Exempt from marketing legislation (no unsubscribe link required). Open rates of 60-80% — leverage that for brand experience and upselling.
Trigger
A trigger is an event that automatically starts an email: form submission, purchase, birthday, inactivity or a lead score threshold. Trigger-based emails are inherently relevant and form the basis of email automation.
U
Unsubscribe
Unsubscribe is the opt-out process. Legally required: clear link, one-click process, immediate processing. Optionally offer a preference center as an alternative. Below 0.5% per campaign is healthy.
Unsubscribe rate
The unsubscribe rate: (unsubscribes / delivered emails) x 100. Below 0.5% is normal. Above 1%: you're sending too often, content isn't relevant, or expectations don't match. Analyze which campaigns cause spikes.
V
Email verification
Email verification checks whether addresses are valid and active using tools like ZeroBounce or NeverBounce. It checks for syntax errors, spam traps and inactive accounts. Verify on import, before major campaigns and for lists that haven't been emailed in 6+ months.
W
Warming
See IP warming.
Welcome series
A welcome series is an automated sequence (3-5 emails, 1-2 weeks) sent immediately after signup: welcome email, brand introduction, social proof and first offer. Highest engagement of all automations — 50%+ open rate. The most important automation to set up first.
Whitelist
A whitelist is the opposite of a blacklist: approved senders whose emails are always let through. Recipients who add you to their contacts are effectively whitelisting you. Actively ask for this in your welcome email.
Workflow
A workflow is an automation flow with multiple steps and branches: triggers, wait periods, if/then conditions and CRM updates. Where a drip campaign is linear, a workflow branches based on behavior. Modern ESPs offer visual drag-and-drop builders.