Glossary
Email glossary
The terms you meet when you send email from an application or a marketing tool, defined in a sentence or two. Where a term has its own guide, the definition links to it.
- Alignment
- The DMARC rule that the domain checked by SPF or the domain that signed with DKIM must match the domain in the From address. Relaxed alignment accepts the same organisational domain; strict alignment needs an exact match.Read the guide
- ARC (Authenticated Received Chain)
- A standard that lets forwarding services, such as mailing lists, record the authentication results they saw before they changed a message, so the final receiver can take them into account.
- ARF
- Abuse Reporting Format: the standard message format used to send complaint reports, for example through a feedback loop.
- BIMI
- Brand Indicators for Message Identification: a DNS record that points to a brand logo some mail clients show next to messages. It requires DMARC at p=quarantine or p=reject, and some providers also require a mark certificate.
- Blocklist
- A list of IP addresses or domains that have sent spam, published by an organisation and used by receiving servers to refuse or filter mail. Also called a blacklist, DNSBL or RBL.
- Bounce
- A receiving server's answer that a message could not be delivered, either during the SMTP conversation or later as a delivery status notification.Read the guide
- Complaint
- A recipient marking a message as spam. Providers count complaints against the sender, and a high complaint rate leads to filtering or blocking.
- Deferral
- A temporary refusal (a 4xx reply) that asks the sender to try again later. The sender's server queues the message and retries.
- DKIM
- DomainKeys Identified Mail: a signature added to each message with a private key; receivers check it against the public key published in the sending domain's DNS.Read the guide
- DKIM selector
- The name that tells receivers which DKIM key to fetch. The key lives at selector._domainkey.domain, so one domain can publish several keys.
- DMARC
- Domain-based Message Authentication, Reporting and Conformance: a DNS policy that requires SPF or DKIM to pass in alignment with the From domain, says what to do with mail that fails, and asks for reports.Read the guide
- DMARC aggregate report
- A daily XML summary, sent by receivers to the address in a domain's rua tag, of the messages they saw using that domain and whether each source passed SPF, DKIM and DMARC.
- DNS
- The Domain Name System, which stores the records a domain publishes: addresses, mail servers, and the TXT records used for SPF, DKIM and DMARC.
- Double opt-in
- A sign-up process in which the subscriber confirms the address by clicking a link in a first message before receiving any marketing mail.
- DSN
- Delivery status notification: a message a server sends back to the envelope sender to report that delivery failed or was delayed. Often simply called a bounce message.
- Enhanced status code
- A three-part code such as 5.1.1 that receiving servers add to SMTP replies. The first digit gives the class (2 success, 4 temporary, 5 permanent) and the rest the reason.
- Feedback loop (FBL)
- A service some mailbox providers run that sends the sender a report each time a recipient marks a message as spam, so the sender can stop mailing that person.
- From address
- The sender address shown to recipients. DMARC protects it by requiring the authenticated domains to align with it.
- Hard bounce
- A permanent delivery failure about the recipient, such as a mailbox or domain that does not exist. The address should not be mailed again.Read the guide
- List-Unsubscribe
- An email header that gives mail clients an unsubscribe address. With the List-Unsubscribe-Post header (RFC 8058) a single click unsubscribes the recipient.Read the guide
- Marketing email
- Mail sent to many recipients for promotion or news, such as newsletters and campaigns. It needs consent and an unsubscribe option.
- MTA
- Mail transfer agent: the server software that queues messages, connects to receiving servers, retries temporary failures and records the result of each attempt.
- MTA-STS
- A policy a receiving domain publishes to tell sending servers that they must use TLS with a valid certificate when delivering to it.
- MX record
- The DNS record that names the servers that receive mail for a domain, each with a priority. Without one, senders fall back to the domain's address record, so publish an explicit MX for any domain that receives mail.
- Open tracking
- Counting opens with a small image loaded when a message is displayed. Some mail clients load images automatically or block them, so open counts are an estimate.
- PTR record (reverse DNS)
- The DNS record that maps an IP address back to a host name. Receivers expect a sending IP to have one that matches the host's forward record.
- Return-Path (envelope sender)
- The address in the SMTP MAIL FROM command, where bounces are sent and which SPF checks. It is usually not the From address recipients see.
- Role address
- An address that belongs to a function rather than a person, such as info@, admin@ or support@. It often reaches several people or none.
- Sender reputation
- The judgement mailbox providers keep about a sending IP and domain, built from complaints, bounces, engagement and authentication, which affects whether mail is delivered to the inbox, filtered to spam or refused.
- SMTP
- Simple Mail Transfer Protocol: the protocol servers use to hand email to each other, and that applications use to submit email to a sending service.
- SMTP relay
- A server that accepts mail from your application over SMTP, usually with a user name and password or API key, and delivers it onward.
- Soft bounce
- A temporary delivery failure, such as a full mailbox or a busy server. The sender retries; only repeated failures over a long period justify removing the address.Read the guide
- Spam trap
- An address that is never used to sign up for anything, or an abandoned address reused by a provider, kept to detect senders with poorly collected or outdated lists.
- SPF
- Sender Policy Framework: a TXT record listing the servers allowed to send mail for a domain, checked against the envelope sender of each message.Read the guide
- STARTTLS
- An SMTP command that upgrades a plain connection to an encrypted TLS connection before any message content is sent.
- Suppression list
- The list of addresses a sender must not mail, or must not mail with a given kind of message, because they bounced, complained or unsubscribed.Read the guide
- Transactional email
- Mail sent because of something a single recipient did or needs, such as a receipt, a password reset or a shipping notice.
- Warm-up
- Raising sending volume gradually on a new IP address or domain, so mailbox providers can build a reputation from a small, well-received start.Read the guide
- Webhook
- An HTTP request a service sends to your application when something happens, for example when a message is delivered or bounces.
From definitions to sending
Mail.Land shows the DNS records your domain needs and tells you when each one verifies.