Email & deliverability, in plain language
41 terms you’ll run into while sending, authenticating and troubleshooting email, each explained with what it means and what to do about it.
Authentication
The DNS records that prove your email really comes from you.
7 terms
SPF (Sender Policy Framework)
A DNS TXT record listing which mail servers are authorized to send email on behalf of your domain. Receiving servers check it to catch spoofed senders.
Read moreDKIM (DomainKeys Identified Mail)
A cryptographic signature added to outgoing email that lets receiving servers verify the message really came from your domain and wasn’t altered in transit.
Read moreDMARC
A DNS policy that tells receiving servers what to do with mail that fails SPF/DKIM alignment (none, quarantine, or reject), and provides aggregate reports on who’s sending as your domain.
Read moreBIMI (Brand Indicators for Message Identification)
A standard that displays your verified brand logo next to authenticated emails in supporting inboxes, requiring a valid DMARC policy at enforcement.
Read moreReturn-Path
The address bounce notifications are sent to, which can differ from the visible "From" address — used by receiving servers to route delivery failures.
Read moreDMARC alignment
The DMARC check that the domain verified by SPF or DKIM matches the domain in the visible From address. Passing SPF or DKIM isn’t enough on its own; the domains have to align.
Read moreReverse DNS (PTR record)
A DNS record that maps an IP address back to a hostname. Receiving servers check that a sending IP has a matching reverse DNS entry, and treat mail without one as suspicious.
Read moreDeliverability
What decides whether your email reaches the inbox.
13 terms
Sender reputation
A score mailbox providers maintain for a sending domain or IP based on engagement, complaints and bounces, which determines whether your mail reaches the inbox, spam, or gets rejected outright.
Read moreInbox placement
Where a delivered email actually lands — primary inbox, promotions tab, or spam folder — as opposed to whether it was accepted by the server at all.
Read moreIP warm-up
The practice of gradually increasing send volume from a new domain or IP so mailbox providers build trust in your sending pattern before you reach full volume.
Read moreDedicated IP
A sending IP address used only by your account, giving you full control over its reputation — as opposed to a shared IP, whose reputation depends on every sender using it.
Read moreEngagement rate
How often recipients open, click, or otherwise interact with your email — a primary signal mailbox providers use to decide future inbox placement.
Read moreSpam trap
An email address with no real owner, used by mailbox providers and blocklists to catch senders with poor list hygiene — sending to one is a strong negative signal.
Read moreBlocklist (or blacklist)
A list of IPs or domains flagged as sources of spam or abuse, checked by many receiving servers before accepting mail — landing on one can tank deliverability until you’re delisted.
Read moreDomain warm-up
Gradually increasing how much email you send from a new or newly used domain, so mailbox providers can build a reputation for it before you send at full volume.
Read moreShared IP
A sending IP address used by many senders on the same email platform. Everyone shares its reputation, which the platform keeps healthy by policing what goes out through it.
Read moreBulk sender requirements
The rules Gmail and Yahoo enforce, since 2024, on anyone sending around 5,000 or more emails a day to their users: full authentication, one-click unsubscribe and a low spam rate.
Read morePostmaster Tools
Free dashboards from mailbox providers, most importantly Google Postmaster Tools, that show how they see your domain: spam rate, reputation, authentication results and delivery errors.
Read moreList hygiene
The ongoing work of keeping your contact list accurate and engaged: removing invalid addresses, bounces and long-inactive contacts so you only send to people who can and want to receive your mail.
Read moreSunset policy
A rule for when you stop emailing unengaged contacts, for example after six months with no opens or clicks, to protect engagement rates and sender reputation.
Read moreSending & delivery events
What happens to an email after you hit send.
10 terms
Hard bounce
A permanent delivery failure — the address or domain doesn’t exist. Hard bounces should be suppressed immediately and never retried.
Read moreSoft bounce
A temporary delivery failure — a full mailbox or a momentarily unreachable server. Soft bounces are worth retrying a few times before giving up.
Read moreComplaint (FBL / feedback loop)
A signal from a mailbox provider that a recipient marked your email as spam. Feedback loops relay these back to senders so the address can be suppressed.
Read moreSuppression list
A list of addresses your platform will never send to again — populated automatically by hard bounces, complaints and unsubscribes to protect your sender reputation.
Read moreMTA (Mail Transfer Agent)
The software that routes email between servers over SMTP — the underlying engine that actually delivers a message from sender to recipient.
Read moreWebhook
An HTTP callback your application receives in real time when an event happens — a delivery, open, click, bounce or complaint — instead of polling for status.
Read moreMX record
A DNS record that specifies which mail servers are responsible for receiving email on behalf of a domain, and in what priority order.
Read moreSMTP relay
A service that accepts email from your application or server over SMTP and delivers it for you, so existing software can send through an email platform just by changing its mail settings.
Read moreThrottling (rate limiting)
When a mailbox provider temporarily limits how much mail it accepts from you, usually answering with a temporary error, because your volume jumped suddenly or your reputation is uncertain.
Read moreEmail validation
Checking whether an email address is correctly formatted, belongs to a domain that accepts mail, and isn’t disposable, before you add it to a list or send to it.
Read moreEmail types & metrics
The kinds of email you send and how to measure them.
11 terms
Transactional email
Email triggered by a specific user action and containing information they need as a result — a receipt, password reset, or login alert.
Read moreMarketing email
Email sent to promote something — a product update, offer, or newsletter — typically requiring recipient consent and a working unsubscribe mechanism.
Read moreOpen rate
The share of delivered emails recorded as opened. Increasingly imprecise due to privacy features that pre-fetch images regardless of a real open.
Read moreClick-through rate (CTR)
The share of delivered emails that received at least one click on a tracked link — a stronger engagement signal than open rate.
Read moreList-Unsubscribe header
An email header that lets recipients unsubscribe in one click directly from their inbox, without opening the message — required by major providers for bulk senders.
Read moreDouble opt-in
A signup flow that requires a subscriber to confirm their email address via a confirmation link before they’re added to a marketing list.
Read moreSegment
A dynamically evaluated group of contacts matching a set of property or event filters, used to target campaigns and automations precisely.
Read moreBounce rate
The share of sent emails that couldn’t be delivered. A high hard-bounce rate is one of the fastest ways to damage your sender reputation.
Read moreUnsubscribe rate
The share of delivered marketing emails that led to an unsubscribe. Some unsubscribes are healthy, but a rising rate means your content or frequency isn’t matching what people signed up for.
Read morePreheader text
The short preview line most inboxes show after the subject line. It’s taken from the first text in the email, so if you don’t set it deliberately, readers see whatever comes first.
Read moreDrip campaign
A series of emails sent automatically on a schedule or in response to what a contact does, such as an onboarding sequence that starts when someone signs up.
Read morePut it into practice
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