Email Deliverability Glossary

Email warmup is the practice of gradually increasing sending volume from a new or dormant mailbox while generating positive engagement — opens, replies, and moves out of spam — so that mailbox providers learn to trust the sender and route future messages to the inbox.

Inbox placement is the percentage of your sent emails that land in the recipient’s primary inbox rather than the spam, junk, or promotions folder.

Email deliverability is the ability to reliably land emails in recipients’ inboxes rather than spam folders or being blocked entirely.

A cold email is an unsolicited message sent to a recipient with no prior relationship, typically for sales outreach, recruiting, or partnerships.

Sender reputation is the trust score mailbox providers assign to a sender based on their IP address and domain sending history.

Domain reputation is the portion of sender reputation tied to your sending domain rather than the IP address.

Sender Score is a reputation rating from 0 to 100, produced by Validity (formerly Return Path), that estimates the health of a sending IP address based on 30 days of activity.

Email authentication is a set of DNS-based protocols — chiefly SPF, DKIM, and DMARC — that let mailbox providers verify a message genuinely came from the domain it claims and was not altered in transit.

SPF (Sender Policy Framework) is an email authentication standard that lets a domain owner publish, in DNS, the list of mail servers authorized to send email on the domain’s behalf.

DKIM (DomainKeys Identified Mail) is an email authentication method that adds a cryptographic signature to each message using a private key.

DMARC (Domain-based Message Authentication, Reporting & Conformance) is a policy layer built on SPF and DKIM.

BIMI (Brand Indicators for Message Identification) is a standard that lets a verified sender display its brand logo next to emails in supporting inboxes.

Bounce rate is the percentage of sent emails that could not be delivered and were returned by the receiving server.

A hard bounce is a permanent email delivery failure — the address is invalid, the domain does not exist, or the server has permanently rejected the message.

A soft bounce is a temporary email delivery failure — the recipient’s mailbox is full, the server is down, or the message is too large.

Complaint rate is the percentage of delivered emails that recipients mark as spam.

A feedback loop (FBL) is a service offered by mailbox providers that notifies a sender whenever a recipient marks their email as spam.

An email blacklist (also called an RBL or DNSBL) is a real-time database of IP addresses and domains flagged for sending spam.

A spam trap is an email address created or repurposed by mailbox providers and anti-spam groups specifically to catch senders with poor list hygiene.

Greylisting is an anti-spam technique where a receiving server temporarily rejects email from an unfamiliar sender with a "try again later" response.

A seed list is a set of test email addresses across major providers — Gmail, Outlook, Yahoo, and others — used to measure inbox placement.

Email throttling is the controlled pacing of outbound email — limiting how many messages are sent per hour or day — to stay within mailbox providers’ rate limits and avoid the volume spikes that trigger spam filtering.

Spam folder placement is when a mailbox provider accepts your email but routes it to the spam or junk folder instead of the inbox.

A mailbox provider (MBP) is a company that operates email inbox services for end users — such as Gmail, Outlook/Microsoft 365, Yahoo, and Apple iCloud Mail.

An MX (Mail Exchange) record is a DNS record that tells the internet which mail servers are responsible for receiving email for a domain.

IMAP (Internet Message Access Protocol) is a standard protocol for reading email stored on a mail server.

An ESP (Email Service Provider) is a company that provides the infrastructure to send email at scale — handling delivery, authentication, and sending reputation on your behalf.

The CAN-SPAM Act is a 2003 US law that sets the rules for commercial email.