Reach the inbox. Not the spam folder.

The science of email deliverability

Inbox placement is determined by a continuous evaluation that every major mailbox provider runs on every inbound message. The evaluation weighs roughly forty signals; the dominant ones in 2026 are sender reputation (60-70 percent of the score), authentication pass status (15-20 percent), content quality (10-15 percent), and per-recipient engagement history (5-10 percent). Optimizing for deliverability means continuously instrumenting and improving each of these inputs.

Sender reputation

Sender reputation is the rolling score mailbox providers assign your sending IP and domain based on historical complaint rate, bounce rate, engagement quality, and blocklist appearances. Reputation tier is asymmetric: losing it is fast (one spam-trap hit or one bad campaign can drop a tier overnight), regaining it is slow (typical recovery from Bad tier to High tier takes 30-90 days of careful sending). Inbox OSS instruments reputation at the IP level, domain level, and content fingerprint level — all three are surfaced in dashboards and exportable as a Slack feed.

Authentication alignment

Authentication alignment is the technical foundation. Without aligned DKIM, SPF, and DMARC, you cannot achieve high inbox placement at any provider regardless of how good your reputation otherwise is. Alignment specifically means: the domain in SPF (from MAIL FROM) and the domain in DKIM (from the signing record) must both match the domain in the From header. Inbox OSS configures DKIM at the per-domain level (each customer domain gets its own 2048-bit RSA keypair stored encrypted), enforces SPF inclusion via CNAME delegation, and provides DMARC monitoring with aggregate report parsing.

Content quality

Content evaluation runs through provider-specific spam classifiers (Google has “Universal Mail Classification,” Microsoft has SmartScreen, Yahoo has Symantec Email Security). The classifiers look at: URL reputation (links to high-reputation domains help, links to low-reputation domains hurt), HTML structure (excessive nested tables, heavy CSS-in-style, missing plain-text alternative all hurt), trigger words (CAPS-heavy subjects, promotional language clusters, suspicious financial terms), and image-to-text ratio (all-image emails get filtered). Inbox OSS includes a SpamAssassin score in the template editor so you can see and improve your score live as you compose.

Engagement quality

Per-recipient engagement is the long-game signal. Mailbox providers track whether your recipients open, click, reply, and dwell on your emails — and route future mail to the inbox or spam folder accordingly. Recipients who have consistently engaged with your domain in the past have high “engagement reputation” specific to your domain; those who have not engaged have low engagement reputation. This is why list hygiene matters so much: keeping unengaged subscribers on your list drags down your engagement reputation and pulls down placement for the engaged subscribers too. Suppress unengaged subscribers proactively; do not wait for them to complain.