Operating Since 2015
Email deliverability is dense with terminology. Many of these terms get used interchangeably in casual conversation but mean different things technically. The reference below captures the 50 that come up most often in production operations, organised by the layer they belong to: authentication, delivery, reputation, list quality, and compliance.
For a deeper treatment of any term, the full 700+ term glossary at cloudserverforemail.com/glossary.html includes operational examples and cross-references. This quick reference is for keeping handy during incident response, vendor calls, or onboarding new team members — the moments when "what does DMARC alignment actually mean again?" comes up.
Authentication
The five terms that determine whether mail authenticates correctly at receiving MTAs. Alignment is the one most operators get wrong: SPF and DKIM can both pass technically while DMARC still fails because the From: domain does not align with the authenticated domain.
SPF — Sender Policy Framework. DNS record listing authorized sending IPs.
DKIM — DomainKeys Identified Mail. Cryptographic message signature.
DMARC — Policy using SPF/DKIM results to accept/quarantine/reject.
BIMI — Brand logo in inbox (requires DMARC at enforcement).
PTR/rDNS — Reverse DNS. IP → hostname. Must match EHLO.
Alignment — From: domain matches SPF envelope or DKIM signing domain.
Delivery
The mechanics of getting messages from your MTA to the receiving MTA. PowerMTA-specific terms appear here because PowerMTA is the dominant production MTA for high-volume senders; the accounting log and pattern-list concepts are PowerMTA terms that have spread into general usage.
MTA — Mail Transfer Agent. Sends/receives email via SMTP.
Deferral (4xx) — Temporary rejection. Retry appropriate.
Hard bounce (5xx) — Permanent failure. Suppress immediately.
Queue depth — Messages waiting in MTA spool for delivery.
Deferral rate — % messages temporarily rejected per ISP. Target <5%.
Throughput — Messages delivered per hour.
max-smtp-out — PowerMTA: concurrent connections per IP per ISP.
max-msg-rate — PowerMTA: messages per hour per IP per ISP.
Accounting log — PowerMTA per-message delivery record (CSV).
dsnDiag — ISP diagnostic text in PowerMTA accounting log.
smtp-pattern-list — Rules mapping ISP response text to bounce types.
Virtual MTA — Named sending IP configuration in PowerMTA.
MX rollup — Groups ISP MX hosts into one throttle bucket.
Reputation
What ISPs assess based on your sending behaviour. Postmaster Tools v2 launched September 2025 retiring the four-tier Domain Reputation gauge in favour of spam-rate-driven signals. SNDS at Microsoft uses per-IP Green/Yellow/Red status. Both feed the same underlying judgement: should this sender's mail go to inbox.
IP reputation — ISP's assessment of sending IP behavior history.
Domain reputation — Gmail's assessment of sending domain (prioritized over IP).
Reputation tier — High/Medium/Low/Bad (Gmail); Green/Yellow/Red (SNDS).
Postmaster Tools — Google's sender reputation dashboard. postmaster.google.com
SNDS — Microsoft's per-IP reputation dashboard.
FBL — Feedback Loop. ISP complaint notification (Yahoo, Comcast).
JMRP — Microsoft complaint feedback program.
ARF — Abuse Reporting Format. Standard FBL complaint email format.
List Quality
The terms that surface during deliverability post-mortems. Complaint rate is the threshold metric: 0.10% per the February 2024 Gmail/Yahoo Bulk Sender Requirements is the documented limit, but healthy programmes run under 0.05% steady state.
Complaint rate — % marking mail as spam. Gmail threshold: 0.10%.
Spam trap — Address that should never receive email.
Hard bounce rate — % addresses permanently failing. Target <0.5%.
Double opt-in — Consent confirmed by email click. Strongest legal basis.
Suppression list — Addresses never to email: bounced, unsubscribed, complained.
Engagement — Opens, clicks, replies. Positive reputation signal.
Re-engagement — Campaign confirming interest of inactive subscribers.
Warming — Gradual volume increase on new IPs to build reputation.
Compliance
The legal framework that bounds operational decisions. EU jurisdictions (GDPR, AEPD enforcement in Spain, ePrivacy Directive) impose stricter consent requirements than US (CAN-SPAM). Canadian CASL sits between the two with a hard opt-in requirement.
CAN-SPAM — US opt-out email law. Commercial email only.
GDPR — EU data protection. Requires legal basis for processing.
CASL — Canadian anti-spam. Opt-in required before sending.
DPA — Data Processing Agreement. Required under GDPR Article 28.
Legitimate interest — GDPR legal basis for B2B email (with balancing test).
Data residency — Where subscriber data is physically stored.