Email Deliverability

Acceptance rate measures the percentage of emails accepted by receiving mail servers without bouncing. Unlike deliverability rate, acceptance rate includes all non-bounced emails regardless of whether they reach the inbox or spam folder. A high acceptance rate indicates your emails pass initial server-level checks but does not guarantee inbox placement.

Backscatter is automated bounce messages sent to innocent third parties whose email addresses were forged by spammers. When spammers send mass emails using fake sender addresses, mail servers generate bounce notifications that flood the forged address owner's inbox. This form of collateral spam damage can severely impact email deliverability and sender reputation.

A blocklist (also called a blacklist or denylist) is a real-time database of IP addresses and domains identified as sources of spam, fraud, or other malicious email activity. When your sending IP or domain appears on a blocklist, email providers may reject your messages outright or route them directly to spam folders, severely impacting your ability to reach recipients.

Email bounce rate is the percentage of sent emails that could not be delivered to recipients' inboxes. It is calculated by dividing the number of bounced emails by the total number of emails sent, then multiplying by 100. Bounce rate is a critical metric for email deliverability - high bounce rates damage sender reputation and can lead to blacklisting.

An email complaint occurs when a recipient marks an email as spam or junk using their email client's reporting feature. The complaint rate measures the percentage of recipients who flag your emails as unwanted, typically calculated per campaign or over time. High complaint rates damage sender reputation and can lead to inbox providers blocking or filtering your messages entirely.

DDoS (Distributed Denial of Service) is a cyberattack where multiple compromised systems flood a target server or network with overwhelming traffic, causing service disruption. In email infrastructure, DDoS attacks can cripple mail servers, prevent legitimate email delivery, and compromise communication systems. These attacks often use botnets to generate massive volumes of malicious traffic that exhaust server resources and bandwidth.

Email deliverability refers to the ability of your emails to successfully reach recipients' inboxes rather than being filtered to spam folders or blocked entirely. It encompasses multiple factors including sender reputation, authentication protocols, content quality, and list hygiene that collectively determine whether mailbox providers accept and properly place your messages. High deliverability rates are essential for email marketing success, as even well-crafted campaigns fail if they never reach their intended audience.

Domain reputation is a score that email providers assign to your sending domain based on the historical behavior of emails sent from that domain. Unlike IP reputation which can change when you switch email providers, domain reputation follows your brand everywhere. It is increasingly the primary factor email providers use to determine inbox placement.

Email bounce handling is the process of managing and responding to emails that fail to reach their intended recipients. It involves detecting bounced emails, categorizing them by type (hard or soft bounce), taking appropriate actions such as removing invalid addresses, and implementing strategies to reduce future bounces. Effective bounce handling is essential for maintaining sender reputation and email deliverability.

Email deliverability is the measure of how successfully your emails reach recipients' inboxes rather than being filtered to spam folders, blocked by ISPs, or bouncing back entirely. It encompasses a complex interplay of factors including sender reputation, email authentication protocols (SPF, DKIM, DMARC), content quality, sending infrastructure, and list hygiene practices. High deliverability rates are essential for email marketing ROI, customer communication, and maintaining a positive sender reputation with major email providers.

Email fatigue occurs when recipients become overwhelmed, disengaged, or annoyed by receiving too many emails from a sender or in general. This psychological response leads subscribers to ignore, delete, or unsubscribe from emails they previously valued, ultimately resulting in declining engagement metrics and increased spam complaints that can damage sender reputation.

Email infrastructure refers to the complete technical foundation that enables sending, receiving, routing, and managing email communications. It encompasses mail servers (SMTP, IMAP, POP3), DNS records for authentication (SPF, DKIM, DMARC), IP addresses, email service providers, security protocols, and monitoring systems. A robust email infrastructure ensures reliable message delivery, protects against spam and phishing, and maintains sender reputation for effective email marketing and business communications.

Email rate limiting is a control mechanism used by email service providers and mailbox providers to restrict the number of emails that can be sent or received within a specific time period. This practice helps prevent server overload, reduces spam, and protects both senders and recipients from abuse. For email marketers, understanding and respecting rate limits is crucial for maintaining deliverability and avoiding temporary blocks or permanent blacklisting.

Email throttling is the practice of controlling the rate at which emails are sent to manage delivery speed and protect sender reputation. It involves limiting the number of emails sent per hour or day to avoid triggering ISP spam filters and rate limits. Proper throttling helps maintain high deliverability rates by preventing your sending IP from being flagged as a source of bulk or spam mail.

Email warmup is the process of gradually increasing email sending volume from a new or dormant email address to establish a positive sender reputation. This systematic approach signals to email providers that you are a legitimate sender, not a spammer. Without proper warmup, sudden high-volume sending can trigger spam filters and damage your domain reputation.

A feedback loop (FBL) is a service provided by email providers that notifies senders when recipients mark their emails as spam. When a subscriber clicks the spam button in their inbox, the email provider sends a report back to the sender, allowing them to remove that person from their mailing list and investigate why the complaint occurred.

Greylisting is an anti-spam technique where mail servers temporarily reject emails from unknown senders, returning a "try again later" response. Unlike blocklisting, greylisting is a temporary measure that tests whether the sending server follows proper email protocols. Legitimate mail servers will retry delivery after the delay, while spammers typically move on to their next target.

A hard bounce is a permanent email delivery failure that occurs when an email cannot be delivered due to a permanent reason, such as an invalid email address, non-existent domain, or blocked recipient. Hard bounces should be removed from your list immediately.

Inbox placement rate (IPR) is the percentage of sent emails that successfully land in recipients' primary inbox rather than spam folders, promotions tabs, or being blocked entirely. It is a more accurate measure of email deliverability than delivery rate, which only counts whether an email was accepted by the receiving server.

IP reputation is a score that email providers assign to the IP address used for sending emails. This score reflects the sending history and trustworthiness of that IP address. A good IP reputation means your emails are more likely to reach the inbox, while a poor reputation can cause your emails to be blocked or sent to spam folders.

IP warming is the gradual process of establishing a positive sending reputation for a new or dormant IP address by systematically increasing email volume over time. This practice signals to email service providers (ESPs) and inbox providers that your IP is a legitimate sender, not a spammer. Proper IP warming is essential when launching new email programs, migrating to dedicated IPs, or resuming sending after extended periods of inactivity.

IP warmup is the strategic process of gradually increasing email sending volume from a new or dormant IP address to establish a positive sender reputation with Internet Service Providers (ISPs). This technique involves sending small batches of emails initially and progressively scaling up over weeks, allowing mailbox providers to recognize the IP as legitimate rather than flagging it as a potential spam source.

List hygiene is the practice of regularly cleaning and maintaining your email list to remove invalid, inactive, and problematic email addresses. Good list hygiene improves email deliverability, reduces bounce rates, protects sender reputation, and ensures you are only sending to people who want to receive your emails.

The postmaster is the administrative email account responsible for managing an email domain's mail system operations. This account receives automated notifications about bounced messages, delivery failures, and abuse reports, while also handling critical functions like blocklist management, spam filter configuration, and compliance with email standards. Postmaster tools provided by major email providers allow senders to monitor their reputation and diagnose deliverability issues.

A queued email is a message that has been composed and submitted for delivery but is temporarily held in a waiting state before being sent. This happens when the email server cannot immediately process the message due to connectivity issues, rate limiting, server load, or email provider restrictions. Once the underlying issue is resolved, queued emails are automatically processed and sent to their recipients.

Rate limiting is a technique used by servers and email providers to control the number of requests or actions a user can perform within a specified time period. It serves as a protective mechanism against abuse, spam, and Denial-of-Service (DoS) attacks while ensuring fair resource allocation among all users. In email marketing, rate limiting determines how many emails you can send per hour or day to maintain deliverability and sender reputation.

Sender reputation is a score assigned by email service providers to your sending IP address and domain based on your historical email sending behavior and recipient engagement patterns. This score directly determines whether your emails land in the inbox, get filtered to spam, or are blocked entirely. Major mailbox providers like Gmail, Microsoft, and Yahoo each maintain their own reputation scoring systems that evaluate factors such as bounce rates, spam complaints, authentication status, and subscriber engagement.

A shared IP address is an IP that multiple senders use to dispatch their emails through the same email service provider. This arrangement allows smaller businesses to send emails without the cost of maintaining a dedicated IP, but it means that all senders on that IP collectively influence its reputation. If one sender engages in poor practices like sending spam or maintaining dirty lists, the resulting reputation damage affects every other sender sharing that same IP address.

A soft bounce is a temporary email delivery failure that occurs when an email cannot be delivered due to a temporary issue, such as a full mailbox, server downtime, or message size limits. Soft bounces may succeed on retry.

Spam refers to unsolicited bulk email messages sent without recipient consent, typically for commercial, fraudulent, or malicious purposes. Also known as junk mail, spam emails are distributed en masse to large recipient lists with the goal of maximizing responses through sheer volume. These messages range from legitimate but unwanted marketing to phishing attempts and scams that threaten both individual users and email ecosystem integrity.

A spammer is an individual or entity that sends unsolicited bulk emails to recipients who have not opted in to receive them. Spammers typically distribute irrelevant, deceptive, or malicious content to large numbers of email addresses, often using automated tools and harvested email lists. Their activities violate anti-spam laws like CAN-SPAM and GDPR, and they are frequently blocklisted by email service providers to protect users from unwanted messages.

A suppression list is a database of email addresses that should never receive emails from your organization. It includes addresses that have bounced, unsubscribed, filed spam complaints, or been manually added due to legal or compliance reasons. Proper suppression list management is essential for maintaining sender reputation and legal compliance.

Email throttling is the practice of limiting the rate at which emails are sent to control delivery speed and prevent overwhelming recipient servers. It can be implemented by senders to protect their reputation or enforced by ISPs and email service providers to manage incoming mail volume and prevent spam.

A whitelist is a list of approved email senders that recipients or email service providers have designated as trusted and safe. Emails from whitelisted senders bypass spam filters and are delivered directly to the inbox, ensuring reliable communication. Whitelisting can occur at the individual recipient level, corporate email server level, or ISP level.