The language of email deliverability, decoded.
Everything you'll run into when working on your email program - bounces, DKIM, sender reputation, throttling, recycling, send-time AI - in plain English, with practical notes on how Seventh Sense addresses each piece.
A
Allowlist (also: whitelist)
A list maintained by an inbox provider, security gateway, or recipient that explicitly permits mail from certain senders. Being on an allowlist gives you preferential treatment but doesn't guarantee inbox placement - engagement still has to back it up.
Authentication
The set of protocols (SPF, DKIM, DMARC) that prove an email actually came from the domain it claims to come from. Without proper authentication, inbox providers can't trust you, and engagement-based filtering treats your sends as suspect.
B
BIMI (Brand Indicators for Message Identification)
A standard that lets verified, well-authenticated senders display their logo next to messages in supporting inbox providers. Requires DMARC enforcement and a Verified Mark Certificate. Cosmetic on its own - but a signal of mature sender practices.
Blocklist (also: blacklist, blocklisting)
A public or private list of IPs and domains that an inbox provider, security gateway, or third party (Spamhaus, SURBL, etc.) considers malicious or spammy. Once you're on one, individual sends from your domain can be silently dropped or sent straight to spam.
Bounce (hard / soft)
Hard bounce: permanent failure - the address doesn't exist or has been permanently blocked. Soft bounce: temporary failure - mailbox full, server down, message too large. Repeated soft bounces from the same address should be treated like hard bounces.
C
Click rate
Clicks divided by emails delivered. Tells you what percentage of people who got your email actually engaged with the content.
Click-through rate (CTR)
Clicks divided by opens. Tells you what percentage of people who opened your email went further. A useful diagnostic for whether your subject line is over-promising or the content is under-delivering.
Cohort
A group of subscribers segmented by a shared attribute - signup date, engagement tier, inbox provider, geography. Cohort-level reporting is how you spot whether a problem is isolated or systemic.
Cold sending
Sending to addresses that haven't engaged with your brand recently (or ever). The fastest way to damage your sender reputation. Inbox providers treat low engagement as a stronger negative signal than they treat the actual content of the message.
D
Deliverability
The percentage of sent emails that actually reach the inbox (not just "accepted by the receiving server"). This is the number that matters - and the one no ESP report shows you directly.
Delivery rate
The percentage of emails that didn't bounce. It does not mean the email made it to the inbox. A 99% delivery rate can still mean 60% of those emails went to spam. The most-cited and most-misunderstood number in email.
DKIM (DomainKeys Identified Mail)
A cryptographic signature added to each email that lets the receiver verify the message wasn't tampered with and actually came from your domain. A foundational piece of email authentication - if you don't have DKIM set up, inbox providers won't take anything else you do seriously.
DMARC (Domain-based Message Authentication, Reporting and Conformance)
A policy layer on top of SPF and DKIM. Tells receivers what to do with mail that fails authentication (none / quarantine / reject) and where to send failure reports. Required for BIMI; strongly recommended for any serious sender.
Domain reputation
The trust score inbox providers assign to your sending domain based on historical performance - engagement, complaints, spam-trap hits, authentication consistency. Domain reputation is increasingly more important than IP reputation. It travels with you, even if you change ESPs.
E
Engagement
The set of signals an inbox provider watches to decide how to treat your future sends: opens, clicks, replies, forwards, "this is not spam," moves out of spam, dwell time, and (negatively) deletions without reading, "mark as spam," and unsubscribes.
Engagement-based filtering
The dominant filtering paradigm at Gmail, Outlook, and most modern inbox providers. Instead of just scanning content for spam keywords, the provider learns from each user's behavior: if recipients delete your emails without opening them, future sends quietly get demoted to Promotions or Spam.
ESP (Email Service Provider)
The platform that handles the actual mechanics of sending - HubSpot, Marketo, Mailchimp, Salesforce Marketing Cloud, etc. Your ESP is the engine. Seventh Sense is the intelligence layer that sits on top.
F
Feedback loop (FBL)
A program (offered by most major inbox providers) that reports back to senders when a recipient marks one of their messages as spam. Lets you suppress the complainant immediately. A "must have" if you're sending at meaningful volume.
From address
The visible "From" your recipient sees. Best practice: a real, identifiable person or named brand address. Avoid no-reply@ - it tells recipients (and increasingly, inbox providers) that you don't want a conversation.
G
Greylisting
A receiving-server tactic where the first delivery attempt from an unknown sender is temporarily rejected. Legitimate MTAs retry; many spam tools don't. Causes a brief delivery delay the first time but is otherwise invisible.
H
Header
The metadata at the top of every email - From, To, Subject, Reply-To, Return-Path, authentication results, server hops, etc. Most of this is hidden in normal email clients but visible via "View original" or "Show all headers" - and it's how deliverability problems get diagnosed.
I
Inbox placement rate
The percentage of delivered emails that actually landed in the primary inbox (not Promotions, not Spam, not "missing"). The single most important number in email performance - and one almost nobody measures directly because traditional reports don't expose it.
Inbox provider (also: mailbox provider, ISP)
The company that runs the recipient's inbox: Google (Gmail / Google Workspace), Microsoft (Outlook / O365), Yahoo, Apple iCloud, Proofpoint, Mimecast, etc. Each one has its own filtering logic, and what works at Gmail may flop at O365.
IP reputation
The trust score assigned to the specific IP address(es) sending your mail. Used to be the dominant signal. Still matters - but most modern filtering also factors in domain reputation, which is harder to "wash" by changing infrastructure.
L
List hygiene
The discipline of removing or suppressing addresses that hurt deliverability: hard bounces, repeated soft bounces, spam complainants, role addresses (info@, sales@), and chronically unengaged subscribers. Most marketing teams under-do it because nobody wants to "shrink the list."
M
MTA (Mail Transfer Agent)
The software that actually moves email between servers - Postfix, Exim, SendGrid, AWS SES, etc. Your ESP runs an MTA (or rents one). Most marketers never need to know what theirs is.
O
Open rate
Opens divided by emails delivered. Used to be the headline metric. After Apple Mail Privacy Protection (2021), open rates became noisy and partially inflated - so they're directional now, not authoritative. Useful for trend analysis, not absolute truth.
Opt-in (single vs. double)
Single opt-in: someone submits a form, they're on the list. Double opt-in: they submit a form AND click a confirmation link in a follow-up email. Double opt-in produces smaller but dramatically more engaged lists. Required in some jurisdictions; recommended everywhere.
Orchestration
Coordinating which emails go to which people, when, and with what frequency caps - across nurtures, batch sends, triggered flows, and one-off campaigns. The opposite of a static content calendar that ignores list overlap and engagement timing.
P
Permission-based marketing
Sending only to people who explicitly asked to hear from you. The baseline expectation under GDPR, CASL, and increasingly enforced by inbox providers via engagement signals. The opposite of "we bought a list."
Postmaster Tools
Free dashboards from Google (postmaster.google.com) and Microsoft (SNDS) that show domain owners how their sends are performing - reputation, spam rate, authentication errors, IPv6 stats. Limited to data the provider chooses to share, and lagged by 24-48 hours.
Promotions tab (Gmail)
The "non-spam, non-priority" bucket in Gmail. Not as bad as Spam - you weren't blocked - but visibility plummets. Most marketing email lives here. Some marketers actively try to land here (it's where Gmail users expect promotions); others fight to get out.
R
Re-engagement campaign
A one-shot campaign aimed at "winning back" unengaged subscribers - "We miss you!", a discount, a content offer. Captures a small slice of dormant people but often spikes complaint rates and damages reputation in the process.
Recycling
The practice of temporarily suppressing low-engagement subscribers from active sends, then gradually reintroducing them when they're more likely to engage. A core Seventh Sense capability - it lets you protect deliverability without losing addresses.
Reply-to
The address that gets used when a recipient hits "Reply." Often (and ideally) different from the From address. A real, monitored reply-to signals to inbox providers that you're a legitimate two-way correspondent, not a one-way broadcaster.
Reputation (sender)
The cumulative trust score inbox providers assign to your sending IPs and domain based on past behavior. Built slowly with consistent, engaged sending. Destroyed quickly with bursts of low-engagement volume or spam complaints. Hardest part of email to game.
S
Sender Policy Framework (SPF)
A DNS record that lists which IPs are authorized to send mail on behalf of your domain. The oldest of the three authentication standards (SPF, DKIM, DMARC). Necessary but not sufficient on its own.
Send-time optimization
Determining the best moment to deliver each individual email based on that person's behavior, time zone, and engagement patterns. Done well, lifts opens and clicks substantially. Done poorly (or as a global "best time to send"), barely moves the needle.
Spam complaint rate
The percentage of recipients who hit "Report spam" or "Mark as junk." Gmail and Yahoo now publish a hard threshold (0.3%); going over it puts you in the penalty box. Even staying under it, complaints are a leading indicator of reputation damage.
Spam folder
The inbox provider's quarantine bucket. Once you're there, you're effectively invisible - and getting out is much harder than staying out. Engagement signals from the small percentage of people who fish you out of spam matter disproportionately for your recovery.
Spam trap
An email address designed to identify spammers. Pristine traps are addresses never used by a real person (so any sender there got the address from scraping or buying lists). Recycled traps are abandoned mailboxes turned into traps (so senders there have poor list hygiene). Hitting either tanks your reputation.
Subdomain
A "child" domain used to segment sending reputation. E.g., `mail.yourbrand.com` for marketing, `txn.yourbrand.com` for transactional, `notify.yourbrand.com` for product. Lets reputation problems in one stream not contaminate another.
Sunset policy
A documented rule for when an address stops getting marketing email (e.g., "no opens in 180 days = removed from active sending"). Most teams don't have one written down; the ones that do tend to have healthier programs.
Suppressed list (also: suppression)
The set of addresses your ESP will not send to: unsubscribers, hard bounces, complainants, and (ideally) chronically unengaged people. Suppression is one-way - it's the cleanest form of list hygiene.
T
Throttling
Deliberately slowing down the rate of sending to avoid tripping inbox provider rate limits or appearing to send in a burst pattern. Critical for big sends or new IPs. Most ESPs do basic throttling; few do it well per-provider.
Tabs (Gmail)
Gmail's category bucketing: Primary, Social, Promotions, Updates, Forums. Marketing email typically routes to Promotions; transactional email typically to Updates. Where Gmail puts your mail is a function of engagement, content classification, and historical sender behavior.
U
Unsubscribe rate
The percentage of recipients who opt out per send. A healthy program has a low and stable rate. Sudden spikes correlate with reputation damage - the message that triggered the spike was likely also generating spam complaints.
W
Warmup (IP / domain warmup)
The practice of gradually ramping up send volume on a new IP or domain so inbox providers can build a reputation profile based on legitimate engagement. Skipping warmup is one of the fastest ways to land on a blocklist.
Now that you know the terms, see what they look like in your program.
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