Email glossary

A feedback loop is a mechanism where major ISPs notify senders when recipients mark messages as spam. Provides direct visibility into which sends generated complaints so the sender can suppress those addresses and investigate root causes.

A hard bounce is a permanent email delivery failure. The address does not exist, the domain has no mail server, or the mailbox was deactivated. Hard-bounced addresses must be removed from your list immediately.

A soft bounce is a temporary email delivery failure such as inbox full, server down, or recipient temporarily blocked. ESPs typically retry on a schedule and suppress after 3 consecutive failures. Counts toward your bounce rate.

A suppression list is the universal do-not-send list maintained by your ESP. Once an address is on it (via hard bounce, unsubscribe, spam complaint, or manual addition), your ESP refuses to send to it regardless of which campaign or list contains it.

An ESP (Email Service Provider) is a platform that handles the sending of bulk email on your behalf. Mailchimp, SendGrid, Klaviyo, ActiveCampaign, Brevo, and HubSpot are all ESPs. Each manages sending infrastructure, list management, deliverability, and reporting.

An MTA (Mail Transfer Agent) is the software that sends and relays email between mail servers using SMTP. Examples: Postfix, Sendmail, Exim, MS Exchange. Most senders use an ESP rather than running their own MTA. The ESP operates the MTA on their behalf.

An MX (Mail Exchange) record is a DNS entry that specifies which mail server is responsible for receiving email for a domain. Without an MX record, the domain cannot receive email at all. That is one of the checks our verification engine runs.

Bounce rate is the percentage of sent emails that failed to deliver. Includes both hard bounces (permanent, address does not exist) and soft bounces (temporary, server unavailable, inbox full). Anything above 2% is a deliverability concern.

DKIM (DomainKeys Identified Mail) cryptographically signs outbound email with a private key, allowing receiving servers to verify the message came from the claimed domain and was not tampered with in transit.

DMARC (Domain-based Message Authentication, Reporting & Conformance) is a policy layer that builds on SPF and DKIM. It tells receiving servers what to do with mail that fails authentication and requests aggregate reports back to the domain owner.

Double opt-in (also called confirmed opt-in) is a subscriber-acquisition method where new signups must click a confirmation link in an email before being added to the list. Improves list quality at the cost of 20-40% conversion drop-off.

Throttling is when a receiving server intentionally limits the rate at which it accepts messages from a particular sender, usually as a defensive measure against suspected spam. Surfaces as soft bounces, slower delivery, or temporary errors.

IP warm-up is the gradual ramp from zero to full sending volume on a fresh sending IP address. New IPs have no reputation. ISPs treat them with suspicion until volume and engagement patterns establish trust. Typical curve runs 4 to 6 weeks of doubling roughly every 3 to 5 days.

Sender reputation is the trust score that ISPs (Gmail, Microsoft, Yahoo) assign to your sending domain and IP, based on authentication, bounce rate, complaint rate, engagement, and consistency. Determines whether your mail reaches the inbox or spam folder.

Single opt-in (SOI) is a subscriber-acquisition method where users are added to the list immediately upon form submission, without an email confirmation step. Faster growth, more typos and abuse without real-time verification protection.

SMTP (Simple Mail Transfer Protocol) is the standard protocol that mail servers use to send and relay email. Defined in RFC 5321. Email verification uses SMTP commands (RCPT TO) to check mailbox existence without sending an actual message.

SPF (Sender Policy Framework) is a DNS-based email authentication mechanism that lists the servers and IPs allowed to send mail on behalf of a domain. Receiving servers check the SPF record on every inbound message.

RCPT TO is the SMTP command that specifies the recipient of an email message. Receiving servers reply with whether they will accept mail for that address. That is exactly how email verification probes mailbox existence without sending a real message.

TLS (Transport Layer Security) encrypts the SMTP conversation between two mail servers, protecting the message in transit. Now standard across most modern mail providers. Opportunistic TLS upgrades the connection when both sides support it.

Transactional email is system-generated mail triggered by a user action: password resets, signup confirmations, order receipts, shipping notifications. Different deliverability profile from marketing email. Typically higher engagement and lower complaint rates.