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3 termsBIMI
BIMI (Brand Indicators for Message Identification) is an email specification that lets a domain display its logo next to authenticated messages in supporting inboxes. It requires DMARC at enforcement and, at Gmail, a verified mark certificate.
Bounce Rate
Bounce rate is the percentage of sent emails that were returned as undeliverable, either permanently (hard bounces) or temporarily (soft bounces). A high bounce rate signals a poor-quality list and damages sender reputation.
Bulk Sender Requirements
Bulk sender requirements are the rules Gmail, Yahoo and Microsoft apply to high-volume senders: SPF, DKIM and DMARC authentication, a spam rate below 0.3 percent, and one-click unsubscribe for marketing mail. Gmail and Outlook set the threshold at about 5,000 messages a day.
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1 termD
5 termsDKIM (DomainKeys Identified Mail)
DKIM (DomainKeys Identified Mail) is an email authentication standard, defined in RFC 6376, in which the sending domain adds a cryptographic signature to each message. Receivers verify it with a public key published in the domain's DNS.
DMARC
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is an email standard that ties SPF and DKIM results to the visible From domain. A domain owner publishes a DNS policy saying how receivers should treat failing mail and where to send reports.
DNS Record
A DNS record is an entry in the Domain Name System that maps a domain name to data such as an IP address, a mail server or a text policy. Email relies on several record types, including MX, TXT, CNAME and PTR, to route and authenticate mail.
Domain Alignment
Domain alignment is the DMARC rule that the domain authenticated by SPF or DKIM must match the domain in the message's visible From address. Relaxed alignment accepts the same organizational domain; strict alignment requires an exact match.
Domain Reputation
Domain reputation is the trust a mailbox provider assigns to a sending domain, based on how mail authenticated with that domain has performed over time. It follows the domain across IP addresses and services, and is a major factor in inbox placement.
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6 termsEmail Authentication
Email authentication is the set of standards, mainly SPF, DKIM and DMARC, that let a receiving server verify that a message really comes from the domain it claims. It protects against spoofing and gives mailbox providers a reliable identity to attach reputation to.
Email Blocklist
An email blocklist is a published list of IP addresses or domains associated with spam, which receiving servers query in real time to decide whether to reject or filter mail. Most are distributed through DNS as DNSBLs, with Spamhaus the best-known operator.
Email Deliverability
Email deliverability is the ability of a sender's messages to reach the recipient's inbox rather than the spam folder or a rejection. It depends on authentication, sender reputation, list quality, sending volume and how recipients engage with the mail.
Email Headers
Email headers are the structured fields at the top of every message, such as From, To, Subject, Date, Message-ID, Received and Authentication-Results, that describe who sent it, how it traveled and whether it passed authentication. Their format is defined in RFC 5322.
Email Service Provider (ESP)
An email service provider (ESP) is a company that sends email on behalf of other businesses, providing sending infrastructure, IP addresses, APIs, templates, list management and delivery reporting. The term usually refers to marketing or transactional platforms rather than personal mailbox providers.
Email Warmup
Email warmup is the practice of gradually increasing sending volume from a new or dormant domain, mailbox or IP address so that mailbox providers can build a positive reputation for it. Sending at full volume from day one looks like spam and often triggers filtering.
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1 termG
1 termH
1 termI
3 termsIMAP
IMAP (Internet Message Access Protocol) is the standard protocol mail clients use to read and manage messages stored on a server. The current version, IMAP4rev2, is defined in RFC 9051 and keeps folders, flags and read status in sync across devices.
Inbox Placement
Inbox placement is the share of delivered emails that land in the recipient's primary inbox rather than the spam folder, a promotions tab or quarantine. It is a stricter measure than delivery rate, which only counts messages the receiving server accepted.
IP Reputation
IP reputation is the trust score mailbox providers and blocklist operators assign to the IP address that sends an email, based on the volume, complaints and spam seen from it. A poor IP reputation can cause throttling, spam placement or outright rejection.
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1 termM
2 termsMessage-ID
Message-ID is an email header that holds a globally unique identifier for a single message, written as a string in angle brackets with an at sign, such as a random value followed by the sender's domain. Mail clients use it to thread replies and detect duplicates.
MX Record
An MX (mail exchanger) record is a DNS record that names the servers responsible for accepting email for a domain, each with a priority number. Sending servers look it up to know where to deliver mail addressed to that domain.
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2 termsOne-Click Unsubscribe
One-click unsubscribe is a standard, defined in RFC 8058, that lets a recipient opt out with a single action in their mail client, which sends an HTTPS POST to the sender. Gmail and Yahoo have required it for bulk marketing mail since 2024.
Open Tracking
Open tracking is a method of recording when an email is opened by embedding a tiny, uniquely addressed image, often called a tracking pixel, that loads from the sender's server. Privacy features such as Apple Mail Privacy Protection and image proxies have made the data unreliable.
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2 termsReturn-Path
The Return-Path is the envelope sender address of an email, set by the SMTP MAIL FROM command and recorded in a header by the final receiving server. Bounces are sent there, and SPF authenticates its domain.
Reverse DNS (PTR Record)
Reverse DNS maps an IP address back to a host name using a PTR record in a special reverse zone. Mail receivers check that a sending IP has a PTR record whose host name resolves back to the same IP, known as forward-confirmed reverse DNS.
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11 termsSecondary Sending Domain
A secondary sending domain is a separate domain, often a close variant of the company's main domain, used for outbound email such as cold outreach. It keeps any reputation damage from affecting the primary domain used for everyday business mail.
Sender Reputation
Sender reputation is the trust score that mailbox providers assign to a sender, based on the history of its domain and IP addresses. It is shaped by spam complaints, bounces, spam trap hits, authentication and recipient engagement, and it strongly affects inbox placement.
Sending Limits
Sending limits are caps on how many emails or recipients a mailbox, domain or IP address can send in a given period. Mailbox providers enforce hard limits to prevent abuse, and senders set their own lower limits to protect reputation.
Shared IP vs. Dedicated IP
A shared IP is a sending IP address used by many customers of an email provider, so their traffic and reputation are pooled. A dedicated IP is used by one sender alone, giving full control of its reputation but requiring steady volume to build and keep it.
SMTP
SMTP (Simple Mail Transfer Protocol) is the internet standard for sending email between servers and from mail clients to their outgoing server. Defined in RFC 5321, it moves each message through a sequence of commands and numeric reply codes.
Soft Bounce
A soft bounce is a temporary delivery failure, signaled by a 4xx SMTP reply, where the receiving server defers a message because of a full mailbox, a busy server, rate limiting or a temporary policy block. The sender may retry, but repeated soft bounces should be treated as permanent.
Spam Complaint Rate
Spam complaint rate is the share of delivered messages that recipients mark as spam. Gmail and Yahoo require senders to keep it below 0.3 percent, and Google recommends staying under 0.1 percent, making it one of the most closely watched deliverability metrics.
Spam Filter
A spam filter is software that decides whether an incoming email is delivered to the inbox, placed in a spam or junk folder, or rejected. Modern filters combine authentication checks, sender reputation, blocklists, content analysis and machine learning based on user feedback.
Spam Trap
A spam trap is an email address that no real person uses to read mail, kept by a mailbox provider or blocklist operator to catch senders with poor list practices. Mailing one signals bought, scraped or stale data and can lead to blocklisting.
SPF (Sender Policy Framework)
SPF (Sender Policy Framework) is an email authentication standard, defined in RFC 7208, that lets a domain publish a DNS TXT record listing the servers allowed to send mail on its behalf. Receivers check the connecting server against that list.
Suppression List
A suppression list is a set of email addresses a sender must never mail again, typically people who unsubscribed, complained, or hard-bounced. Sending tools check it before every send, so suppressed contacts stay excluded even if they reappear in a new import.