Knowledge Base
Reference
Email Glossary
90 terms covering email authentication, deliverability, DNS, infrastructure, and compliance.
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Authentication(16)
- Aggregate Report
- Daily XML reports sent by mailbox providers to the address in your DMARC rua tag, summarizing authentication results for all emails from your domain.Related: DMARC, Forensic Report
- Alignment
- The requirement that the domain in the visible From header matches the domain used in SPF (envelope sender) or DKIM (d= domain) authentication, either exactly (strict) or including subdomains (relaxed).Related: DMARC, SPF, DKIM
- ARC
- Authenticated Received Chain — a protocol that preserves email authentication results across forwarding hops, preventing legitimate forwarded mail from failing DMARC.Related: DMARC, Forwarding
- BEC
- Business Email Compromise — a type of attack where criminals impersonate executives or vendors to trick employees into making fraudulent payments or revealing sensitive data.Related: Phishing, Spoofing
- BIMI
- Brand Indicators for Message Identification — a standard that allows brands to display their logo next to authenticated emails in supporting inboxes.Related: DMARC, VMC
- Canonicalization
- The process of converting email content to a standard format before DKIM signing, handling variations in whitespace and header formatting.Related: DKIM
- DKIM
- DomainKeys Identified Mail — a cryptographic signature added to outgoing emails that proves the message hasn't been altered and comes from an authorized sender.Related: SPF, DMARC, Selector
- DMARC
- Domain-based Message Authentication, Reporting & Conformance — a policy that tells receiving servers what to do when SPF or DKIM checks fail, and provides reporting on authentication results.Related: SPF, DKIM, Alignment
- Forensic Report
- Detailed reports sent for individual email authentication failures to the DMARC ruf address — contains message details but is not widely supported.Related: DMARC, Aggregate Report
- MTA-STS
- Mail Transfer Agent Strict Transport Security — a mechanism that enables mail servers to declare their ability to receive TLS-secured connections and specify whether sending servers should refuse to deliver to MX hosts that don't support TLS.Related: TLS, TLS-RPT, STARTTLS
- Phishing
- Fraudulent emails that impersonate legitimate senders to trick recipients into revealing passwords, financial information, or other sensitive data.Related: Spoofing, DMARC
- Selector
- A unique identifier in a DKIM record that allows multiple DKIM keys to exist for the same domain, enabling key rotation and multiple sending services.Related: DKIM
- SPF
- Sender Policy Framework — a DNS TXT record that lists which IP addresses and servers are authorized to send email on behalf of your domain.Related: DKIM, DMARC, DNS
- Spoofing
- Forging the From address of an email to make it appear to come from a trusted sender, commonly used in phishing attacks — prevented by DMARC.Related: Phishing, DMARC
- TLS-RPT
- TLS Reporting — a standard for reporting failures in TLS connectivity, typically used alongside MTA-STS to monitor email encryption issues.Related: MTA-STS, TLS
- VMC
- Verified Mark Certificate — a digital certificate that validates brand logo ownership, required by some email providers for BIMI logo display.Related: BIMI
DNS & Records(13)
- A Record
- Address record — a DNS record that maps a domain name to an IPv4 address.Related: AAAA Record, DNS
- AAAA Record
- IPv6 Address record — a DNS record that maps a domain name to an IPv6 address.Related: A Record, DNS
- CNAME Record
- Canonical Name record — a DNS record that creates an alias from one domain name to another, often used for DKIM records pointing to an ESP's key server.Related: DNS, DKIM
- DNS
- Domain Name System — the distributed database that translates domain names into IP addresses and stores various record types including those used for email authentication.Related: MX Record, TXT Record, A Record
- FCrDNS
- Forward-Confirmed Reverse DNS — when a PTR record points to a hostname that, when looked up, resolves back to the original IP address, proving ownership.Related: PTR Record, Reverse DNS
- MX Record
- Mail Exchange record — a DNS record that specifies which mail servers are responsible for receiving email for a domain and their priority order.Related: DNS, Priority
- NS Record
- Name Server record — a DNS record that specifies which DNS servers are authoritative for a domain.Related: DNS
- Priority
- A numeric value in MX records that determines the order in which mail servers should be tried, with lower numbers indicating higher priority.Related: MX Record
- Propagation
- The time it takes for DNS changes to spread across the global DNS infrastructure after a record is updated, typically ranging from minutes to 48 hours.Related: DNS, TTL
- PTR Record
- Pointer record — a DNS record used for reverse DNS lookups, mapping an IP address back to a hostname, important for email server reputation.Related: Reverse DNS, FCrDNS
- Reverse DNS
- A DNS lookup that resolves an IP address to a domain name using PTR records, used by mail servers to verify sender legitimacy.Related: PTR Record, FCrDNS
- TTL
- Time To Live — the duration (in seconds) that a DNS record should be cached before being refreshed from the authoritative server.Related: DNS, Propagation
Infrastructure(11)
- Dedicated IP
- An IP address used exclusively by a single sender, giving them full control over their sending reputation.Related: Shared IP, IP Reputation
- ESP
- Email Service Provider — a company that provides email sending infrastructure, such as SendGrid, Mailchimp, Postmark, or Amazon SES.Related: Transactional Email, Marketing Email
- Forwarding
- When an email is automatically redirected from one address to another, which can break SPF authentication since the forwarding server's IP isn't in the original sender's SPF record.Related: ARC, SPF
- Mail Gateway
- A server that acts as an intermediary for email, often providing filtering, security scanning, or routing between networks.Related: MTA, Relay
- MDA
- Mail Delivery Agent — software that delivers email to a user's mailbox after it's been received by the MTA.Related: MTA, MUA
- MSA
- Mail Submission Agent — a server that accepts email from a user's email client (MUA) and forwards it to an MTA for delivery.Related: MTA, MUA
- MTA
- Mail Transfer Agent — software that transfers email messages between servers using SMTP, such as Postfix, Sendmail, or Exchange.Related: SMTP, MDA, MSA
- MUA
- Mail User Agent — an email client application used to read and send email, such as Outlook, Gmail, or Apple Mail.Related: MTA, MSA
- Open Relay
- A misconfigured mail server that allows anyone to send email through it, commonly exploited by spammers and typically results in blacklisting.Related: Relay, Blacklist
- Relay
- The process of forwarding an email from one mail server to another, or a server configured to accept and forward mail on behalf of other systems.Related: MTA, Open Relay
Protocols & Standards(16)
- Base64
- An encoding method that converts binary data to ASCII text, used for email attachments and some DKIM signatures.Related: MIME
- Envelope Sender
- The email address used in the SMTP 'MAIL FROM' command, which receives bounce notifications and is checked by SPF — may differ from the visible From header.Related: Return-Path, From Header
- From Header
- The visible sender address shown to recipients in their email client — this is what DMARC alignment checks against.Related: Envelope Sender, Alignment
- IMAP
- Internet Message Access Protocol — a protocol for retrieving email that keeps messages on the server and synchronizes across multiple devices.Related: POP3, MUA
- List-Unsubscribe
- An email header that provides a URL or email address for one-click unsubscription, required by many mailbox providers and regulations.Related: CAN-SPAM
- Message-ID
- A unique identifier assigned to each email message, used for threading conversations and tracking delivery.
- MIME
- Multipurpose Internet Mail Extensions — a standard that extends email to support attachments, HTML content, and character sets beyond ASCII.Related: Multipart, Base64
- Multipart
- A MIME message format that contains multiple parts, such as both plain text and HTML versions of the same email, or a message with attachments.Related: MIME
- Opportunistic TLS
- TLS encryption that is attempted but not required — if the receiving server doesn't support TLS, the email is sent unencrypted.Related: TLS, STARTTLS, MTA-STS
- POP3
- Post Office Protocol version 3 — a protocol for retrieving email that typically downloads messages to a single device and removes them from the server.Related: IMAP, MUA
- Return-Path
- The email header that contains the envelope sender address, indicating where bounce notifications should be sent.Related: Envelope Sender, Bounce
- RFC
- Request for Comments — technical documents published by the IETF that define internet standards, including email protocols like SMTP (RFC 5321) and authentication methods.
- SMTP
- Simple Mail Transfer Protocol — the standard protocol for sending email between servers, operating on port 25 (server-to-server) or 587 (submission).Related: MTA, STARTTLS
- STARTTLS
- A command that upgrades a plain-text SMTP connection to an encrypted TLS connection, enabling opportunistic encryption.Related: TLS, SMTP, Opportunistic TLS
Deliverability(24)
- Blacklist
- A list of IP addresses or domains identified as sources of spam, maintained by organizations like Spamhaus, and used by mail servers to block incoming mail.Related: DNSBL, Blocklist
- Blocklist
- The preferred industry term for blacklist — a database of IPs or domains known to send spam or malicious email.Related: Blacklist, DNSBL
- Bounce
- An email that couldn't be delivered and is returned to the sender with an error message explaining the failure.Related: Hard Bounce, Soft Bounce
- Cold Email
- Unsolicited email sent to recipients who haven't opted in, often for sales outreach — carries high deliverability risk and legal implications.
- Deferral
- When a receiving server temporarily rejects an email with a request to try again later, often indicated by a 4xx SMTP response code.Related: Throttling, Soft Bounce
- Deliverability
- The ability of an email to successfully reach recipients' inboxes rather than being blocked, bounced, or filtered to spam.Related: Inbox Placement, Sender Reputation
- DNSBL
- DNS-based Blackhole List — a blacklist that can be queried via DNS to check if an IP address is listed as a spam source.Related: Blacklist, RBL
- Domain Reputation
- The trustworthiness score assigned to a sending domain based on its authentication setup, sending history, and engagement metrics.Related: Sender Reputation
- Feedback Loop
- A service provided by mailbox providers that notifies senders when recipients mark their email as spam, enabling list cleanup.Related: Spam Complaint
- Hard Bounce
- A permanent delivery failure, typically because the recipient address doesn't exist — these addresses should be immediately removed from your list.Related: Bounce, Soft Bounce
- Honeypot
- A hidden email address planted to catch spammers who harvest addresses from websites or purchase lists.Related: Spam Trap
- Inbox Placement
- The percentage of delivered emails that land in the primary inbox rather than the spam folder or other tabs.Related: Deliverability
- IP Reputation
- The trustworthiness score assigned to an IP address based on the quality and legitimacy of email sent from it.Related: Sender Reputation, Dedicated IP
- List Hygiene
- The practice of regularly cleaning email lists by removing invalid addresses, unengaged subscribers, and potential spam traps.Related: Bounce, Spam Trap
- Marketing Email
- Bulk promotional emails sent to lists of subscribers, such as newsletters and promotional campaigns — subject to stricter spam filtering.Related: Transactional Email, ESP
- Preheader
- The preview text that appears after the subject line in inbox views, typically the first line of text content or a hidden snippet at the start of the email.
- RBL
- Realtime Blackhole List — another term for DNSBL, a list of IP addresses known to send spam that can be queried in real-time.Related: DNSBL, Blacklist
- Sender Reputation
- A score assigned to a sending IP or domain based on historical sending behavior, used by mailbox providers to determine email placement.Related: IP Reputation, Domain Reputation
- Soft Bounce
- A temporary delivery failure, such as a full mailbox or server timeout — these may succeed if retried later.Related: Bounce, Hard Bounce
- Spam Complaint
- When a recipient clicks the 'Report Spam' or 'Junk' button in their email client, negatively affecting sender reputation.Related: Feedback Loop, Sender Reputation
- Spam Trap
- An email address used to identify spammers — either a recycled address that hasn't been active for years or a pristine address that was never used for signup.Related: Honeypot, List Hygiene
- Throttling
- When a receiving mail server intentionally slows down or limits the rate of incoming email from a sender, often due to volume concerns or reputation issues.Related: Deferral
- Transactional Email
- One-to-one emails triggered by user actions, such as password resets, order confirmations, and shipping notifications — typically have high deliverability.Related: Marketing Email, ESP
- Warm-up
- The process of gradually increasing email sending volume from a new IP or domain to build reputation with mailbox providers.Related: IP Reputation, Dedicated IP
Metrics(5)
- Bounce Rate
- The percentage of sent emails that couldn't be delivered, calculated as bounces divided by total sends.Related: Bounce, Hard Bounce
- Click Rate
- The percentage of delivered emails where a recipient clicked at least one link, generally a more reliable engagement metric than open rate.Related: Open Rate
- Complaint Rate
- The percentage of delivered emails that recipients marked as spam — major providers recommend staying below 0.1%, with 0.3% being a critical threshold.Related: Spam Complaint, Feedback Loop
- Open Rate
- The percentage of delivered emails that were opened, tracked via a tiny invisible image — becoming less reliable due to privacy features like Apple Mail Privacy Protection.Related: Click Rate
- Unsubscribe Rate
- The percentage of recipients who opted out of future emails after receiving a message.
Compliance & Legal(5)
- CAN-SPAM
- A US law regulating commercial email that requires accurate headers, a physical address, clear identification as an ad, and a working unsubscribe mechanism.Related: GDPR, List-Unsubscribe
- CASL
- Canada's Anti-Spam Legislation — Canadian law requiring express consent before sending commercial email, with specific consent record-keeping requirements.Related: CAN-SPAM, GDPR
- Double Opt-in
- A signup process where new subscribers must confirm their email address by clicking a link in a confirmation email before being added to the list.Related: Single Opt-in, GDPR
- GDPR
- General Data Protection Regulation — an EU law requiring explicit consent for email marketing, data protection, and the right to be forgotten.Related: CAN-SPAM, Double Opt-in
- Single Opt-in
- A signup process where subscribers are added to the list immediately after submitting their email, without a confirmation step.Related: Double Opt-in