Reference
Email deliverability glossary.
Plain-language definitions of the email deliverability and verification terms that decide whether your mail reaches the inbox - each one linked to a deeper guide when you want the full story.
- Blocklist (blacklist)
- A published list of IP addresses or domains a mailbox provider or filter will reject or throttle mail from. Being listed suppresses delivery until you fix the cause and, on most lists, request removal. Email blocklists explained →
- Bounce
- A message the receiving server refused. A hard bounce is permanent (the address or domain doesn't exist) and must be suppressed; a soft bounce is temporary (a full mailbox or a transient server issue) and can be retried. Bounce classification →
- Bounce rate
- The share of sent mail that bounces, usually expressed as a percentage. Under 2% is broadly healthy; a rising rate almost always traces to list hygiene rather than the sending platform. Why bounce rate matters →
- Bulk verification
- Checking an entire list of addresses in one batch, typically by uploading a CSV, rather than one address at a time in real time. Used to clean a list you already have before you send. Bulk email verification →
- Catch-all domain
- A domain configured to accept mail to any address, so the server can't confirm whether a specific mailbox exists. Verification returns these as risky because they can't be positively confirmed over SMTP. What is a catch-all email →
- Confidence score
- A 0-100 signal a verifier attaches to each address, ordering results by how safe they look. It's a graded probability, not a guarantee - useful for prioritizing borderline cases. How verification works →
- Dedicated IP
- A sending IP address used by a single sender, so its reputation reflects only your mail. The opposite of a shared IP, where you inherit the sending behavior of everyone on it. Dedicated vs shared IP →
- Deliverability
- Whether your mail actually reaches the inbox rather than the spam folder or a bounce. It's decided by authentication, sender reputation, engagement, complaints, and list quality - not by content alone. Deliverability guide →
- Disposable email address
- A temporary, throwaway inbox (often from a burner-mail service) used to pass a signup without giving a real address. It usually stops working quickly, so it's worth catching at capture. Role, disposable & free addresses →
- DKIM
- DomainKeys Identified Mail - a cryptographic signature added to each message that lets a receiver confirm it wasn't altered and really came from your domain. One of the three core authentication standards. SPF, DKIM & DMARC →
- DMARC
- A DNS policy that ties SPF and DKIM to the visible From address, tells receivers what to do when a message fails authentication, and reports back to you. Google and Yahoo require a published policy for bulk senders. How to set up DMARC →
- DNSBL
- A DNS-based blocklist - a blocklist queried over DNS in real time as mail arrives, returning whether a sending IP or domain is listed. The technical form most blocklists take. Email blocklists explained →
- Double opt-in
- A signup flow where a subscriber confirms via a link emailed to them before being added to your list. It proves the address is real and the person consented, at the cost of some signups. Single vs double opt-in →
- Email verification
- Checking whether an address is valid and able to receive mail before you send (syntax, domain and MX records, and an SMTP mailbox probe), returning a status, reason, and confidence score. What is email verification →
- Feedback loop (FBL)
- A service some mailbox providers offer that forwards you a report whenever a recipient marks your mail as spam, so you can suppress that address. Registering for FBLs is basic complaint hygiene. Email feedback loops →
- Greylisting
- An anti-spam tactic where a server temporarily refuses mail from an unfamiliar sender on first contact, expecting a legitimate server to retry. It can make a mailbox briefly unverifiable. What is greylisting →
- IP warm-up
- Gradually increasing send volume on a new IP or domain so mailbox providers build trust rather than flagging a sudden spike. Skipping it is a common cause of early deliverability problems. How to warm up an IP →
- List hygiene
- The ongoing practice of verifying, cleaning, and suppressing addresses so you only mail real, engaged recipients. It keeps bounces, complaints, and spam-trap hits low. How to clean a list →
- MX record
- A DNS entry that names the mail servers responsible for receiving a domain's email, each with a priority. A domain with no MX record can't accept mail, so every address at it will bounce. MX record lookup tool →
- Null MX
- A single MX record with an empty target (RFC 7505), used by a domain to declare explicitly that it does not accept email. Mail to a null-MX domain is refused outright. MX record lookup tool →
- One-click unsubscribe
- The List-Unsubscribe and List-Unsubscribe-Post header pair (RFC 8058) that lets a recipient opt out in a single tap from the mail client. Google and Yahoo require it on bulk marketing mail. One-click unsubscribe →
- Postmaster Tools
- Google's free dashboard showing your domain and IP reputation, spam-complaint rate, and authentication results as Gmail measures them. The authoritative source for Gmail placement. Emails going to spam in Gmail →
- Reason code
- A short machine-readable label a verifier returns alongside a status, explaining why an address got its verdict - for example a null MX, a full mailbox, or a disposable domain. How verification works →
- Reverse DNS (PTR)
- A DNS record that maps a sending IP back to a hostname. A missing or generic PTR is a common reason a legitimate server lands on infrastructure blocklists like SpamRats. SpamRats blacklist →
- Role-based address
- A shared, function-based mailbox such as info@, sales@, or support@ rather than a person's inbox. These generate more complaints and are worth flagging separately when you verify. Role, disposable & free addresses →
- Sender reputation
- The trust score mailbox providers assign to your sending domain and IP based on complaints, bounces, spam-trap hits, and engagement. It's the single biggest factor in inbox placement. Sender reputation →
- Sender requirements
- The bar Google and Yahoo enforce for bulk senders: SPF, DKIM, and DMARC, one-click unsubscribe, valid reverse DNS, TLS, and a spam-complaint rate under 0.3%. Google & Yahoo sender requirements →
- SMTP
- Simple Mail Transfer Protocol - the language mail servers use to hand off messages. Its numeric response codes (like 250, 550, or 421) tell you whether a message was accepted, rejected, or deferred. SMTP response codes →
- Spam complaint rate
- The share of delivered mail recipients mark as spam. Google requires bulk senders to stay under 0.3% and to avoid ever reaching 0.1% - the loudest negative signal a filter acts on. Spam complaint rate →
- Spam trap
- An address that exists only to catch senders with poor permission or hygiene. Hitting one (a pristine trap that never opted in, or a recycled dead address) signals a dirty list and damages reputation. What are spam traps →
- SPF
- Sender Policy Framework - a DNS record listing which servers are allowed to send mail for your domain. A receiver checks it to confirm the sending server is authorised. SPF, DKIM & DMARC →
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