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IEmail Deliverability Glossary

Inbox Testing (Seed Testing)

Sending email to pre-established seed addresses across multiple ISPs to verify exactly where messages land — inbox, spam, or promotions — before campaigns go live.

Inbox testing (or seed testing) is a method of checking inbox placement by sending your email to a set of pre-established "seed" email addresses across multiple ISPs and email clients, then checking which folder each message landed in.

What inbox testing reveals:

  • Whether you're landing in inbox, spam, or promotions at each major ISP
  • Which ISPs are filtering your mail
  • Whether your authentication records are being read correctly
  • How your email content is interpreted by different filtering systems

Popular inbox testing tools:

  • GlockApps — Extensive seed list, detailed reporting, warm-up tracking
  • Mail-Tester — Quick free spam score and inbox placement check
  • Litmus Spam Testing — Integrates with email design workflow
  • MXToolbox Deliverability — Combined blacklist + authentication + content check

When to run inbox tests:

  • Before starting a new campaign on a newly warmed domain
  • When deliverability metrics drop unexpectedly
  • After any major change to email content, infrastructure, or authentication

Frequently Asked Questions

What is the best tool for inbox placement testing?

GlockApps is the most comprehensive inbox testing tool — it tests placement across 90+ seed addresses at Gmail, Outlook, Yahoo, AOL, iCloud, and major corporate mail providers, shows exact placement (Primary, Promotions, Spam, Missing), and includes authentication and blacklist checks in the report. Mail-Tester.com is a free alternative that provides a quick spam score and basic placement check for one Gmail address. For enterprise testing with larger seed lists and more granular ISP data, Validity's Everest platform is the premium option used by high-volume senders.

How often should I run inbox placement tests during warm-up?

Run a full inbox placement test at the start of warm-up (baseline), again at the end of Week 1, at each significant volume milestone (when you 2x your daily volume), whenever you change email content or templates, and any time campaign metrics (open rates, reply rates) drop unexpectedly. During active warm-up, weekly testing is reasonable. For established senders, monthly or pre-campaign testing catches problems before they affect real audiences. Each test consumes seed address 'freshness' — avoid testing the exact same seed addresses too frequently or results become less reliable.

What should I do if inbox testing shows spam placement?

First, identify which ISPs are filtering your mail — is it Gmail, Outlook, or a specific corporate domain? Check your authentication headers in the test report to confirm SPF and DKIM are passing. Run a content analysis with a spam score tool to identify triggering phrases, problematic links, or HTML issues. Check if your sending IP is blacklisted at any DNSBL. If authentication and blacklists are clean and content looks fine, the issue is likely reputation-based — continue warm-up at current volume without increasing until placement improves. Consider reducing volume temporarily if spam placement is above 40%.

Related Terms

I

Inbox Placement Rate

The percentage of sent emails that land in the recipient's primary inbox. More meaningful than delivery rate — a message in spam is functionally invisible.

D

Deliverability Audit

A systematic review of all factors affecting inbox placement — authentication, reputation, content, list health, and sending patterns.

S

Spam Filter

An algorithm that evaluates incoming email and decides whether to deliver it to the inbox, route it to spam, or block it. Warm-up trains these filters to trust your domain.

D

Deliverability (Email Deliverability)

The ability to successfully land emails in the recipient's primary inbox — not spam, not promotions, not blocked. The ultimate goal of every warm-up effort.

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