Glossary

Alignment

The domain authenticated by SPF or by DKIM being the same one that appears in the From:. It is what DMARC checks. See Alignment.

ARF

Abuse Reporting Format. The format a provider uses to tell us somebody marked a message as spam. It is what produces a complaint event.

Bounce

A message the receiver refused. A hard bounce is permanent, usually because the address does not exist: the address is suppressed. A soft bounce is temporary, like a full mailbox, and shows up here as a deferral.

Cursor

The API's way of paging. Each page carries a next_cursor that is passed to the next one. See Reading the log.

Deferral

The receiver said "not now". It is temporary and we retry; there is nothing to do.

DKIM

DomainKeys Identified Mail. The cryptographic signature proving the message came from who it says. The public key sits in a TXT record in your DNS, under a selector.

DMARC

Domain-based Message Authentication, Reporting and Conformance. The policy telling the recipient what to do when SPF and DKIM do not align, and where to send the reports.

DSN

Delivery Status Notification. The automatic message a server sends back when it cannot deliver. It is what we read to know a bounce happened and why.

Envelope

The pair of addresses servers exchange between themselves, MAIL FROM and RCPT TO, which nobody sees in a mailbox. The MAIL FROM is the Return-Path.

FBL

Feedback Loop. The arrangement with a provider that forwards their users' complaints to us. It is how a complaint reaches us.

HMAC

The signature on every webhook, computed with your secret over the request's body. See Webhooks.

Message-ID

The unique identifier of a message, written in the header and equal to the id the API returns. It is what support asks for.

MX

The DNS record naming the server that receives mail for a domain. The destination MX in the timeline is the server that accepted or refused your message.

Return-Path

The address bounces come back to. Ours by default, and yours if you publish the bounce CNAME.

Selector

The name identifying a DKIM key inside a domain, cs202609 in cs202609._domainkey.example.com. Several selectors can coexist, which is what allows rotating a key without interruption.

SPF

Sender Policy Framework. The TXT record listing who may send on behalf of your domain. There may be only one per domain.

Stream

Transactional or marketing. It separates reputation, the subdomain, the selector and the IPs. See Transactional and marketing.

Suppression

An address we stop sending to. See Suppressions.

Delivery rate

Messages delivered divided by messages sent, over the chosen period. "Delivered" means the recipient's server accepted it, not that the person saw it.

ULID

The kind of identifier we use everywhere. It sorts by time, which is what allows cursor paging with no extra column.

Warm-up

Raising the volume gradually on a new IP or a new domain, so receivers form an opinion from good traffic rather than from a sudden spike.