Glossary · EmailARC (Authenticated Received Chain)ARC is an email standard that lets forwarders and mailing lists record the original authentication results so later receivers can trust them.Glossary · EmailBIMI (Brand Indicators for Message Identification)BIMI lets a brand display its logo next to authenticated email in supporting inboxes, but only if the domain enforces DMARC.Glossary · EmailBulk EmailBulk email is a large volume of near-identical messages sent to many recipients at once, such as newsletters, which mailbox providers treat with stricter rules.Glossary · EmailCAN-SPAM ActThe CAN-SPAM Act is a US law setting rules for commercial email, including accurate headers, a visible opt-out and a physical postal address in each message.Glossary · EmailCatch-All AddressA catch-all address is a mailbox setting that accepts mail sent to any address at a domain, including ones that do not exist, and delivers it to one inbox.Glossary · EmailCC and BCCCC copies an email to additional recipients who are visible to everyone, while BCC copies recipients invisibly so other recipients cannot see their addresses.Glossary · EmailDKIM (DomainKeys Identified Mail)DKIM signs outgoing email with a private key and publishes the public key in DNS so receivers can verify the message was not altered.Glossary · EmailDKIM Key RotationDKIM key rotation is the practice of replacing the key pair used to sign outgoing email on a regular schedule while keeping old signatures verifiable during the changeover.Glossary · EmailDKIM SelectorA DKIM selector is a label in a DKIM signature that identifies which public key in DNS verifies it, allowing a domain to publish several keys and rotate them.Glossary · EmailDMARC (Domain-based Message Authentication)DMARC lets a domain tell receivers what to do with mail that fails SPF or DKIM alignment, and sends back reports about who is using the domain.Glossary · EmailDMARC AlignmentDMARC alignment requires that the domain authenticated by SPF or DKIM matches the domain in the visible From address.Glossary · EmailDMARC Reports (RUA and RUF)DMARC reports are feedback from mail providers: aggregate RUA reports summarise authentication results, while forensic RUF reports cover single failures.Glossary · EmailDNSBL (DNS Blocklist)A DNSBL is a DNS-queryable list of IP addresses or domains reported for spam or abuse, which mail servers check to decide whether to reject a connection.Glossary · EmailDomain Alignment (DMARC)Domain alignment is the DMARC requirement that the domain verified by SPF or DKIM match the domain in the visible From header, in strict or relaxed mode.Glossary · EmailDouble Opt-InDouble opt-in is a signup method where a new subscriber must click a confirmation link in an email before being added to a mailing list.Glossary · EmailEmail AliasAn email alias is an extra address that forwards to an existing mailbox, letting one inbox receive mail for several names without separate accounts.Glossary · EmailEmail Bounce (Hard and Soft)An email bounce is a failed delivery notice from a mail server, with a 4xx or 5xx SMTP code showing whether the failure is temporary or permanent.Glossary · EmailEmail ClientAn email client is an application that connects to mail servers to send, receive and organise messages, either installed on a device or run in a browser.Glossary · EmailEmail Feedback Loop (FBL)An email feedback loop is a service where a mailbox provider reports back to the sender when a recipient marks a message as spam.Glossary · EmailEmail ForwardingEmail forwarding automatically redirects incoming mail from one address to another, often at a different provider, which can break SPF and DMARC checks.Glossary · EmailEmail HeaderAn email header is the metadata block of a message listing sender, recipient, date and the servers it passed through, useful for tracing delivery.Glossary · EmailEmail Open Tracking PixelAn email tracking pixel is a tiny invisible image in a message; when the client loads it the sender's server logs an open, though results are unreliable.Glossary · EmailEmail SpoofingEmail spoofing is forging the sender address of a message so it appears to come from a trusted person or domain, possible because SMTP does not verify it.Glossary · EmailEnvelope Sender (MAIL FROM)The envelope sender is the address given in the SMTP MAIL FROM command, used for routing bounces and for SPF checks, and separate from the visible From header.Glossary · EmailFrom HeaderThe From header is the sender address and display name shown in an email client; it is set by the message author and is not verified by SMTP itself.Glossary · EmailGreylistingGreylisting is an anti-spam technique in which a mail server temporarily rejects mail from an unknown sender, expecting legitimate servers to retry later.Glossary · EmailHard BounceA hard bounce is a permanent email delivery failure, such as a nonexistent address or a blocked domain, which should not be retried.Glossary · EmailIMAP (Internet Message Access Protocol)IMAP is a protocol that lets email clients read and manage messages stored on a server, keeping folders and read state in sync across all your devices.Glossary · EmailIP Warm-UpIP warm-up is the practice of gradually increasing email volume from a new IP address or domain so mailbox providers learn to trust it before heavy sending.Glossary · EmailList HygieneList hygiene is the routine cleaning of a mailing list, removing invalid, inactive and risky addresses to keep bounce rates low and sender reputation healthy.Glossary · EmailList-Unsubscribe HeaderThe List-Unsubscribe header is an email header that exposes a machine-readable unsubscribe link, letting clients show a one-click unsubscribe button.Glossary · EmailMail ServerA mail server is a system that sends, receives, routes and stores email for one or more domains, typically combining an MTA, a delivery agent and IMAP access.Glossary · EmailMailing ListA mailing list is a managed group of addresses where one message sent to the list address is distributed to every subscriber, for discussion or announcements.Glossary · EmailMDA (Mail Delivery Agent)An MDA is the component that takes a finished incoming message from the MTA and stores it in the recipient's mailbox, applying filters such as Sieve rules.Glossary · EmailMTA (Mail Transfer Agent)An MTA is server software that accepts email over SMTP and routes it onward to the next server or to the final delivery agent, such as Postfix or Exim.Glossary · EmailMTA-STS (SMTP MTA Strict Transport Security)MTA-STS is a standard that lets a domain require TLS and a valid certificate when other servers deliver mail to it, preventing downgrade attacks.Glossary · EmailMUA (Mail User Agent)A MUA is the program a person uses to compose, read and organise email, such as Outlook, Thunderbird, Apple Mail or a webmail interface.Glossary · EmailMX Record (Mail Exchanger)An MX record names the mail servers that accept email for a domain and ranks them by priority, with the lowest number tried first.Glossary · EmailPGP (Pretty Good Privacy)PGP is an encryption and signing system for email and files using public and private key pairs, with trust built through user-managed keys, not authorities.Glossary · EmailPOP3 (Post Office Protocol version 3)POP3 is an older email retrieval protocol that downloads messages from a server to one device, usually removing them from the server afterwards.Glossary · EmailReply-To HeaderThe Reply-To header names the address replies should go to when it differs from the From address, so answers reach a monitored inbox or help desk.Glossary · EmailS/MIME (Secure/Multipurpose Internet Mail Extensions)S/MIME is a standard for signing and encrypting email using X.509 certificates issued by certificate authorities, supported natively by many email clients.Glossary · EmailSender ReputationSender reputation is the trust score mailbox providers assign to a sending IP address and domain, based on complaints, bounces, engagement and authentication.Glossary · EmailSMTP (Simple Mail Transfer Protocol)SMTP is the standard protocol mail clients and servers use to send email and to relay it between servers, traditionally on TCP ports 25, 465 and 587.Glossary · EmailSMTP RelayAn SMTP relay is a server that accepts email from a client or application and forwards it onward, often to improve deliverability or centralise sending.Glossary · EmailSoft BounceA soft bounce is a temporary email delivery failure, such as a full mailbox or an overloaded server, that the sending server will normally retry.Glossary · EmailSpam TrapA spam trap is an email address used to catch senders with poor list practices; mail arriving at it signals scraped, purchased or unmaintained lists.Glossary · EmailSPF (Sender Policy Framework)SPF is a DNS TXT record listing which servers may send email for your domain, letting receivers reject or flag forged senders.Glossary · EmailSPF FlatteningSPF flattening replaces include mechanisms in an SPF record with the underlying IP addresses so the record stays within the 10-lookup limit.Glossary · EmailTLS-RPT (SMTP TLS Reporting)TLS-RPT is a standard where sending mail servers report to a domain how often TLS negotiation and MTA-STS checks succeeded or failed when delivering to it.Glossary · EmailTransactional EmailTransactional email is automatically triggered mail about a specific user action, such as a receipt or password reset, rather than promotional content.Glossary · EmailWebmailWebmail is an email client that runs in a web browser and reads mail directly from the provider's servers, such as Gmail, Outlook.com or Roundcube.