Email Deliverability Glossary
51 terms behind inbox placement, defined in plain English - with what each one means in practice and the mistake people most often make with it.
Authentication
A DNS record that lists which servers are allowed to send email for a domain.
The rule that an SPF check may trigger at most 10 DNS lookups, or it fails with a permerror.
Replacing the includes in an SPF record with the IP ranges they resolve to, to stay under the 10 lookup limit.
A cryptographic signature added to each message and checked against a public key published in DNS.
A name that tells the receiver which DKIM public key to look up for a given signature.
A policy that tells receivers what to do with mail that fails SPF and DKIM alignment, and where to send reports.
The p= tag of a DMARC record: none monitors, quarantine sends failures to spam, reject refuses them.
Daily XML summaries from receivers showing which sources sent mail as your domain and whether it passed.
The requirement that the domain verified by SPF or DKIM matches the domain in the visible From address.
The address bounces are sent to, set during the SMTP conversation, and the domain SPF is checked against.
A way for intermediaries such as mailing lists and forwarders to preserve the original authentication results.
A DNS record that maps a sending IP address back to a hostname.
The greeting a sending server uses at the start of an SMTP conversation to identify itself.
A policy that tells sending servers to deliver mail to your domain only over validated TLS.
A DNS record that asks sending servers to report failures to deliver mail to your domain over TLS.
Encryption of the SMTP connection between mail servers.
A DNS record that names the servers that receive mail for a domain, in priority order.
The time it takes for a changed DNS record to be seen by resolvers around the internet.
Reputation
A mailbox provider's running assessment of how trustworthy a sender is, based on its past mail.
The standing of a sending domain with mailbox providers, independent of the IP it sends from.
The standing of the IP address your mail is sent from.
A published list of IPs or domains that receivers use to refuse or filter mail.
An email address that exists only to catch senders with poor list practices.
The share of delivered messages that recipients mark as spam.
A service where a mailbox provider reports back when a recipient marks your message as spam.
Google's free dashboard showing how Gmail sees your domain: spam rate, authentication and compliance.
A sending IP address used by many customers of the same provider.
A sending IP address used only by one sender.
Gradually raising the sending volume of a new IP so providers can build trust in it.
Gradually building the sending history of a new domain or mailbox before running real volume.
Delivery
Whether your email reaches the recipient's inbox, rather than the spam folder or being rejected.
Where an accepted message actually lands: the inbox, a category tab such as Promotions, or spam.
A set of test mailboxes at different providers used to see where a message lands.
Gmail's category tab for commercial and bulk-looking mail, separate from the Primary inbox.
A permanent delivery failure, such as an address that doesn't exist.
A temporary delivery failure that the sending server retries.
The share of messages that couldn't be delivered.
A receiving server temporarily slowing or refusing mail from a sender, usually with a 421 or 450 reply.
Deliberately rejecting the first delivery attempt from an unknown sender and accepting the retry.
A domain whose mail server accepts mail for any address, whether or not the mailbox exists.
A shared address like info@ or support@ rather than a named person's mailbox.
Checking whether an address is worth sending to before you do.
Keeping a mailing list clean: removing bounces, complainers, inactive and invalid addresses.
Sending
A first email to someone who hasn't asked to hear from you, usually for sales or partnerships.
A domain used only for sending outreach, separate from the company's primary domain.
In Gmail's and Yahoo's rules, a sender of about 5,000 or more messages a day to their users.
A message header that lets mail clients show an unsubscribe button, and the RFC 8058 header that makes it one click.
A tiny image in an email whose load is counted as an open.
Rewriting links so a click passes through a tracking server before reaching the destination.
Simple Mail Transfer Protocol, the standard by which mail servers send email to each other.
A company that sends email on your behalf, such as SendGrid, Mailgun or Amazon SES.
Knowing the terms is the start. If your mail is landing in spam, we'll find out which of them is the cause.
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