Email glossary
Every email term, explained without jargon.
Published January 15, 2026 · By RAPID SMTP
- SMTPSimple Mail Transfer Protocol — the standard for sending email between servers.
- SPF (Sender Policy Framework)A DNS record that lists which servers are allowed to send mail for your domain.
- DKIM (DomainKeys Identified Mail)A cryptographic signature added to every outbound email that proves it came from your domain.
- DMARCA DNS policy that tells receivers what to do when SPF or DKIM fail.
- SMTP relayA service that accepts your outbound mail and forwards it to recipient servers on your behalf.
- BYOSMTP (Bring Your Own SMTP)A model where a marketing tool sends through SMTP credentials you already own.
- BounceAn email that could not be delivered. Hard bounces are permanent; soft bounces are temporary.
- Suppression listAddresses you must never send to again — unsubscribes, hard bounces, complaints.
- UnsubscribeThe legally required link recipients click to stop receiving email from you.
- DeliverabilityThe probability that your email lands in the inbox rather than spam.
- IP warm-upGradually ramping send volume on a new IP so receivers build reputation for it.
- Open ratePercentage of recipients who opened a campaign — tracked via an invisible pixel.
- Click-through rate (CTR)Percentage of recipients who clicked any tracked link in a campaign.
- Transactional emailOne-to-one email triggered by user action: receipts, password resets, order confirmations.
- Bulk emailOne-to-many email — newsletters, promos, announcements — sent to a list.