Glossary
The vocabulary of getting into the inbox
Each definition is one sentence and stands on its own, with the accepted numeric thresholds where they exist. Written to be quoted.
- Suppression listalso: Do-not-email list, Exclusion list
- A suppression list is the set of email addresses a sender is permanently or temporarily barred from contacting, typically comprising hard bounces, spam complaints, and unsubscribes.
- Hard bounce
- A hard bounce is a permanent delivery failure indicating the recipient address does not exist or cannot receive mail, and it should suppress the address immediately and permanently.
- Soft bounce
- A soft bounce is a temporary email delivery failure, such as a full mailbox or an unavailable server, where the address may still be valid and deliverable later.
- Bounce rate
- Bounce rate is the proportion of sent messages that fail to deliver, and it should be kept below 5% — above 10%, sending accounts are commonly suspended.
- Healthy: under 5%Warning: 5–10%Critical: above 10%
- Complaint ratealso: Spam complaint rate, Abuse rate
- Complaint rate is the proportion of delivered messages that recipients mark as spam, and it must stay below 0.1% — Gmail's bulk-sender rules treat 0.3% as the enforcement line.
- Healthy: under 0.1%Warning: 0.1–0.5%Gmail enforcement: 0.3%Critical: above 0.5%
- RFC 8058 (one-click unsubscribe)also: List-Unsubscribe-Post, One-click unsubscribe
- RFC 8058 defines the List-Unsubscribe-Post header that lets a mail client unsubscribe a recipient with a single click, and Gmail and Yahoo require it from anyone sending 5,000 or more messages per day.
- DKIMalso: DomainKeys Identified Mail
- DKIM is an email authentication method that attaches a cryptographic signature to outgoing messages so receiving servers can verify the message genuinely came from the claimed domain and was not altered in transit.
- SPFalso: Sender Policy Framework
- SPF is a DNS record listing the servers authorised to send email for a domain, allowing receiving servers to reject messages sent from unauthorised sources.
- DMARC
- DMARC is a policy layer built on SPF and DKIM that tells receiving servers what to do with messages failing authentication, and sends the domain owner reports on those failures.
- Amazon SES sandbox
- The Amazon SES sandbox is the restricted default state of a new SES account, in which messages can only be sent to verified addresses and daily volume is capped until AWS grants production access.
- Sandbox daily cap: 200 messages / 24 hoursSandbox send rate: 1 message / secondTypical approval time: under 24 hours
- Bring-your-own provider (BYO sending)also: BYO SMTP, Bring your own ESP
- Bring-your-own provider is a model in which the marketing platform holds no sending infrastructure and instead dispatches every message through the customer's own SendGrid, Amazon SES, or similar account.
- Data hygienealso: List cleaning, List hygiene
- Data hygiene is the ongoing practice of removing invalid, duplicate, and unreachable contacts from a mailing list so that sends reach real recipients and bounce rates stay within provider tolerances.
- Behavioural cohort
- A behavioural cohort is a group of contacts clustered by what they have actually done — open and click patterns, recency, and engagement trajectory — rather than by static profile fields.
- AI email personalizationalso: Generative email personalization
- AI email personalization is the generation of distinct campaign copy for different audiences from behavioural and contextual signals, ranging from one variant per segment to one variant per individual recipient.
- AEO (Answer Engine Optimization)also: GEO, Generative Engine Optimization, LLM SEO
- Answer Engine Optimization is the practice of structuring web content so that AI answer engines such as ChatGPT, Perplexity, Claude, and Google AI Overviews can extract, cite, and correctly attribute it.
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