Email marketing, in plain English.
Short, clear definitions of the email terms you keep running into. No jargon, no fluff.
Bounce Rate
Bounce rate is the share of your emails that could not be delivered and came back.
Read the definition →Bulk Sender
A bulk sender is anyone who sends a large number of emails, often defined as more than 5,000 a day.
Read the definition →Click-Through Rate (CTR)
Click-through rate is the share of delivered emails where someone clicked a link.
Read the definition →Click-to-Open Rate (CTOR)
Click-to-open rate is the share of people who opened your email and then clicked a link.
Read the definition →Conversion Rate
Conversion rate is the share of people who did what you wanted after getting your email.
Read the definition →Domain Warm-Up
Domain warm-up is slowly raising how much you send from a new domain so email providers learn to trust it.
Read the definition →Double Opt-In
Double opt-in means a new subscriber confirms their email address before they join your list.
Read the definition →Email Authentication
Email authentication is a set of records that prove your email is really from you, not a faker.
Read the definition →Email Deliverability
Email deliverability is how reliably your emails land in the inbox instead of the spam folder.
Read the definition →Email Service Provider (ESP)
An email service provider is a tool that helps you build, send, and track marketing emails.
Read the definition →Hard Bounce
A hard bounce is an email that failed for a permanent reason, like an address that does not exist.
Read the definition →List Hygiene
List hygiene is the habit of keeping your contact list clean and up to date.
Read the definition →Open Rate
Open rate is the share of delivered emails that were opened.
Read the definition →Segmentation
Segmentation is sorting your contacts into groups so you can send each group the right email.
Read the definition →Sender Reputation
Sender reputation is your track record with email providers, which decides how much they trust your mail.
Read the definition →Soft Bounce
A soft bounce is an email that failed for a temporary reason, like a full mailbox.
Read the definition →Spam Complaint Rate
Spam complaint rate is the share of people who marked your email as spam.
Read the definition →Unsubscribe Rate
Unsubscribe rate is the share of people who opted out after getting your email.
Read the definition →
Last updated: June 25, 2026