Reference
Email Deliverability Glossary
Plain-English definitions of 19 essential terms every email administrator and marketer should know.
SPF (Sender Policy Framework)
An email authentication protocol that lets domain owners specify which mail servers are authorized to send email on their behalf. SPF records are published as DNS TXT records and checked by receiving servers to detect forged sender addresses. A properly configured SPF record reduces the chance of your emails being rejected or flagged as spam.
Check your SPF recordDKIM (DomainKeys Identified Mail)
A cryptographic email authentication method that attaches a digital signature to every outgoing message. The receiving server verifies the signature against a public key published in the sender's DNS, confirming the message wasn't altered in transit. DKIM is essential for passing DMARC alignment and building a positive sender reputation.
Check your DKIM configurationDMARC (Domain-based Message Authentication, Reporting & Conformance)
A policy layer that builds on SPF and DKIM to tell receiving servers what to do when authentication fails. DMARC policies can instruct servers to monitor, quarantine, or reject unauthenticated messages. It also provides aggregate and forensic reports so domain owners can monitor abuse of their domain.
Check your DMARC policyDNSBL (DNS-based Blackhole List)
A real-time database of IP addresses and domains known to send spam, queried via DNS lookups. When a mail server receives a connection, it checks the sender's IP against one or more DNSBLs. A listing on a major DNSBL like Spamhaus or Barracuda can cause widespread email delivery failures.
Check 60 blacklists instantlyMX Record (Mail Exchange Record)
A DNS record that specifies which mail servers accept incoming email for a domain. MX records include a priority value, allowing administrators to designate primary and fallback servers. Missing or misconfigured MX records prevent email from reaching its destination entirely.
Look up your MX recordsPTR Record (Pointer Record)
A DNS record used for reverse DNS lookups, mapping an IP address back to a hostname. Many mail servers reject email from IPs without a valid PTR record, because legitimate mail servers almost always have forward-confirmed reverse DNS (FCrDNS). Setting up a correct PTR record is one of the simplest ways to improve deliverability.
Check your reverse DNSSMTP (Simple Mail Transfer Protocol)
The standard protocol for sending email across the internet. SMTP handles the transmission of messages between mail servers and from email clients to outgoing servers. Modern email security depends on extensions like STARTTLS for encryption and various authentication mechanisms built on top of SMTP.
Scan your SMTP securityBounce Rate
The percentage of sent emails that are returned as undeliverable. Hard bounces occur when the recipient address doesn't exist; soft bounces are temporary failures like a full mailbox. A high bounce rate damages sender reputation and can trigger blacklisting, so regular list hygiene is critical.
Sender Reputation
A score assigned to a sending IP address or domain by mailbox providers based on sending behavior. Factors include complaint rates, bounce rates, spam trap hits, blacklist presence, and authentication status. A poor sender reputation causes emails to land in spam or be rejected outright, even if the content is legitimate.
IP Warming
The process of gradually increasing email volume sent from a new IP address to build a positive sending reputation. Mailbox providers are suspicious of new IPs that suddenly send large volumes, so warming involves starting with small batches of emails to engaged recipients and slowly scaling up over several weeks.
Email Authentication
The collective set of protocols (SPF, DKIM, DMARC, and others) used to verify that an email message genuinely comes from the domain it claims to be from. Proper authentication prevents spoofing, improves deliverability, and is now a hard requirement for bulk senders at Gmail and Yahoo.
Test your authenticationBlacklist (Blocklist)
A database of IP addresses or domains identified as sources of spam or malicious email. Being listed on a blacklist causes email delivery failures across all mail servers that reference that list. Common causes include sending spam, hitting spam traps, or having a compromised server.
Check if you're blacklistedWhitelist (Allowlist)
A list of trusted senders whose emails bypass spam filtering. Whitelisting can be done at the server level (by IP or domain) or by individual recipients who mark a sender as safe. While useful for ensuring delivery, whitelisting alone doesn't fix underlying authentication or reputation issues.
Spam Trap
An email address used by anti-spam organizations to identify senders with poor list practices. Pristine traps are addresses that never opted in, catching senders who buy lists. Recycled traps are abandoned addresses repurposed to catch senders who don't remove inactive subscribers. Hitting spam traps severely damages reputation.
Feedback Loop (FBL)
A system where mailbox providers notify senders when recipients mark their emails as spam. When a recipient clicks 'Report Spam,' the provider sends a report back to the sender's registered FBL address. Monitoring feedback loops and promptly unsubscribing complainers is essential for maintaining a healthy sender reputation.
BIMI (Brand Indicators for Message Identification)
A standard that allows organizations to display their brand logo next to authenticated emails in supporting inboxes. BIMI requires a valid DMARC policy at enforcement level (quarantine or reject) and a Verified Mark Certificate (VMC) for full support. It increases brand visibility and recipient trust.
Check your BIMI recordMTA-STS (Mail Transfer Agent Strict Transport Security)
A protocol that enables mail servers to declare their support for TLS-encrypted connections and instructs sending servers to refuse delivery over unencrypted channels. MTA-STS prevents downgrade attacks where an attacker strips the STARTTLS command to intercept emails in plaintext. It requires both a DNS record and a web-hosted policy file.
Check your MTA-STS policyTLS-RPT (TLS Reporting)
A companion standard to MTA-STS that enables domains to receive daily aggregate reports about TLS connection failures when other servers try to deliver email. These reports help administrators identify configuration issues, certificate problems, and potential downgrade attacks affecting their email encryption.
Check your TLS-RPT recordDANE (DNS-Based Authentication of Named Entities)
A protocol that uses DNSSEC to publish TLS certificate information in DNS via TLSA records, allowing sending servers to verify the receiving server's certificate without relying on traditional certificate authorities. DANE provides stronger protection against man-in-the-middle attacks than MTA-STS but requires full DNSSEC deployment.
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