Email Deliverability Glossary
A comprehensive reference of email deliverability terms, protocols, and concepts you need to know.
Blacklist
A database maintained by anti-spam organizations that lists IP addresses and domains known to send unsolicited or malicious emails. If your sending IP or domain appears on a blacklist, your emails are much more likely to be blocked or filtered into spam by receiving mail servers. Regular monitoring and prompt delisting are critical to maintaining good deliverability.
Bounce Rate
The percentage of sent emails that are returned by the receiving mail server. Hard bounces occur when an email address is invalid or does not exist, while soft bounces are temporary failures like a full mailbox. A high bounce rate signals poor list hygiene to email providers and can severely damage your sender reputation, often leading to throttling or blocking.
DKIM (DomainKeys Identified Mail)
An email authentication method that allows the receiving server to verify that an email was actually sent by the domain it claims to be from and has not been altered in transit. DKIM works by attaching a digital signature to every outgoing email, which is validated against a public key published in the sender's DNS records. Proper DKIM configuration is essential for passing DMARC alignment checks.
DMARC (Domain-based Message Authentication, Reporting, and Conformance)
A policy framework that builds on SPF and DKIM to give domain owners control over how receiving servers handle unauthenticated emails. DMARC allows you to specify whether failed emails should be quarantined, rejected, or delivered normally, and provides reporting so you can monitor authentication results across all mail sent from your domain. A properly configured DMARC policy significantly reduces the risk of domain spoofing.
Domain Warm-up
The process of gradually establishing a sending reputation for a new domain by slowly increasing email volume over time. During a domain warm-up, the goal is to demonstrate to email providers that your domain sends legitimate, wanted emails. This typically takes two to four weeks and involves controlled, low-volume sending that increases as your reputation builds. Skipping this step often results in poor inbox placement from day one.
Email Authentication
A set of protocols and techniques used to verify that an email was genuinely sent by the domain it claims to be from. The three main authentication standards are SPF, DKIM, and DMARC. Proper authentication is essential for deliverability because it helps receiving servers distinguish legitimate emails from phishing and spam. Without authentication, even well-crafted emails are likely to be filtered or rejected.
Feedback Loop (FBL)
A mechanism provided by major email providers like Gmail and Outlook that notifies senders when recipients mark their emails as spam. By subscribing to feedback loops, senders can identify recipients who do not want their emails and remove them from their lists, reducing spam complaints and protecting their sender reputation. Not all providers offer FBLs, and the format and availability vary by provider.
Inbox Placement Rate
The percentage of sent emails that successfully reach the primary inbox rather than being filtered into spam, promotions, or other folders. Unlike delivery rate, which only measures whether an email was accepted by the server, inbox placement rate tells you whether recipients can actually see your email. This is the single most important metric for measuring deliverability and is the primary indicator of a successful warm-up campaign.
IP Warm-up
The process of gradually increasing the volume of emails sent from a new or dormant IP address to build a positive sending reputation with email providers. Starting with a small volume and slowly ramping up allows ISPs to observe your sending behavior and categorize your IP as a legitimate sender rather than a spammer. The timeline typically ranges from two to six weeks depending on target volume.
MX Record (Mail Exchanger Record)
A DNS record that specifies the mail server responsible for receiving emails on behalf of a domain. When someone sends an email to your domain, the sending server looks up your MX records to determine where to deliver the message. Properly configured MX records are foundational to email delivery. Multiple MX records with different priorities can be set for redundancy.
PTR Record (Pointer Record)
A DNS record used for reverse DNS lookups, mapping an IP address back to a domain name. Many email providers check that the sending IP address has a valid PTR record that matches the sending domain. A missing or mismatched PTR record can cause emails to be rejected or flagged as suspicious, especially by strict corporate mail servers.
Sender Reputation
A score assigned by email providers based on your historical sending behavior, including bounce rates, spam complaints, engagement rates, and authentication status. A strong sender reputation means your emails are more likely to reach the inbox, while a poor reputation leads to spam filtering or outright blocking. Reputation is built over time through consistent, high-quality sending practices and can be damaged quickly by a single bad campaign.
Spam Score
A numerical value assigned to an email by spam filtering systems that indicates how likely the email is to be spam. Factors that increase spam score include suspicious subject lines, excessive use of images, missing authentication, and content that matches known spam patterns. Lower spam scores mean better chances of inbox placement. Each email provider uses its own scoring algorithm with different weights and thresholds.
Spam Trap
An email address specifically created or repurposed by email providers and anti-spam organizations to catch senders with poor list practices. Pristine spam traps are addresses that were never used by real people, while recycled traps are old addresses that were abandoned and then repurposed. Sending to spam traps is a strong signal of poor list hygiene and can cause severe, long-lasting reputation damage that is difficult to recover from.
SPF (Sender Policy Framework)
An email authentication protocol that allows domain owners to specify which mail servers are authorized to send emails on their behalf. SPF works by publishing a DNS TXT record listing approved sending IP addresses. When a receiving server gets an email, it checks the SPF record to verify the sender is authorized, helping prevent email spoofing. SPF has a 10-lookup limit, so overly complex configurations can cause failures.
Throttling
The practice of limiting the rate at which emails are sent to a particular domain or email provider. Email providers impose rate limits on incoming emails, and sending too many too quickly can trigger temporary blocks or delays. Proper throttling ensures your emails are delivered smoothly without overwhelming receiving servers. Understanding each provider's limits is key to maintaining consistent delivery during high-volume campaigns.
Warm-up
The process of gradually building sender reputation for a new or underused email account, domain, or IP address by sending controlled volumes of email and generating positive engagement signals. During warm-up, automated conversations between trusted mailboxes help establish your account as a legitimate sender, improving inbox placement rates for your real campaigns. A proper warm-up strategy is the foundation of long-term deliverability.
Whitelist
A list of trusted senders maintained by email providers, organizations, or individual users that ensures emails from listed addresses bypass spam filtering. Being whitelisted by a recipient or provider guarantees inbox delivery. While individual whitelisting is common, getting added to provider-level whitelists requires consistently strong sending practices and a proven track record of low complaints and high engagement.