Glossary
Email deliverability glossary
Short definitions of the terms you’ll run into when verifying lists and getting email to the inbox, each with a guide that goes deeper.
A
- Accept-all email
- Another name for a catch-all email: an address on a domain whose mail server accepts messages for any recipient, whether or not the mailbox exists. See catch-all email
B
- Blocklist (blacklist)
- A list of IP addresses or domains that are known to send spam. Mailbox providers check these lists and may reject or filter mail from anything on them. Read more about blocklist
- Bounce rate
- The share of sent emails that come back undelivered. Above about 2% usually points to a list-quality problem. Read more about bounce rate
- Bounced email
- An email the receiving server returned instead of delivering, with an error code explaining why. Read more about bounced email
C
- Catch-all email
- An address on a domain whose mail server accepts messages for any address, whether or not the mailbox exists. Standard verification can’t confirm these addresses. Read more about catch-all email
- Click-to-open rate (CTOR)
- Unique clicks divided by unique opens. It measures how well the content of an email worked for the people who opened it. Read more about click-to-open rate
D
- DKIM
- DomainKeys Identified Mail. A cryptographic signature added to each email, which receivers check against a public key in the sender’s DNS to confirm the message wasn’t altered. Read more about DKIM
- DMARC
- A DNS policy that tells receivers what to do with mail from your domain that fails both SPF and DKIM alignment (nothing, quarantine or reject) and where to send reports. Read more about DMARC
- Disposable email address
- A temporary address from a service like Mailinator, used to get past a sign-up form. How long it keeps mail depends on the provider: some expire stored messages within minutes or days, while the address itself may keep working. Read more about disposable email address
- Double opt-in
- A sign-up that only counts once the person clicks a confirmation link sent to their address. It proves the address is real and that its owner asked to join. Read more about double opt-in
E
- Email append
- Adding email addresses to existing customer records by matching names and postal data against a third-party database. Read more about email append
- Email headers
- The metadata at the top of every email: who sent it, each server it passed through, and SPF, DKIM and DMARC results. Read more about email headers
- Email warm-up
- Building sending volume slowly on a new domain or mailbox so mailbox providers learn to trust it before you send at full volume. Read more about email warm-up
G
- Greylisting
- An anti-spam method where a server temporarily rejects mail from an unfamiliar sender and accepts it on a later retry. It can make verification results come back unknown. Greylisting and unknown results
H
- Hard bounce
- A permanent delivery failure, usually because the address or domain doesn’t exist. Remove the address after the first one. Read more about hard bounce
I
- IMAP
- The protocol email apps use to read and sync mail stored on a server. SMTP sends mail; IMAP retrieves it. Read more about IMAP
- IP warming
- Gradually increasing the volume sent from a new dedicated IP address so it builds a sending reputation. Read more about IP warming
M
- MX record
- The DNS record that says which mail servers accept email for a domain, and in what order to try them. Read more about MX record
R
- Reverse DNS (PTR record)
- A DNS record that maps an IP address back to a hostname. Many receivers distrust mail from sending IPs without one. Read more about reverse DNS
- Role-based email
- An address for a function or team rather than a person, like info@, sales@ or support@. Read more about role-based email
S
- Sender reputation
- How mailbox providers rate a sending domain or IP, based on bounces, spam complaints, spam trap hits and engagement. It decides whether mail reaches the inbox. Read more about sender reputation
- SMTP
- Simple Mail Transfer Protocol, the standard servers use to send and relay email. Email verification also talks SMTP to ask whether a mailbox exists. Read more about SMTP
- SMTP authentication
- Logging in to a mail server before sending through it, so only authorized users can relay mail. Read more about SMTP authentication
- Soft bounce
- A temporary delivery failure, such as a full mailbox or a server that’s down. Mail may go through on a later try. Read more about soft bounce
- Spam trap
- An address used by mailbox providers and blocklist operators to catch senders with poor list practices. Hitting one can get you blocklisted. Read more about spam trap
- SPF
- Sender Policy Framework. A DNS record that lists which servers may send mail for a domain. Read more about SPF
T
- Transactional email
- An email triggered by something a person did, like a receipt, password reset or shipping notice, as opposed to a marketing campaign. Read more about transactional email
V
- VoIP number
- A phone number that runs over the internet instead of a phone line or mobile network. Often used for sign-ups and harder to tie to a real person. Read more about VoIP number
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