Why emails land in spam: reputation, content, and list evidence

If your auth is clean and you're still landing in spam, the problem is usually one of three things: reputation, content, or list hygiene. The frustrating part is that none of them produce a clear error message — your email just disappears into the junk folder while your sending tool reports a successful delivery.

1. Reputation

Mailbox providers score every sending IP and domain. The score is invisible — there's no public dashboard at Gmail or Outlook telling you "you are at 73 today" — but it's what gates inbox placement. A new domain with no history will be filtered aggressively, no matter how perfect its records are.

Warm-up is the only fix. Send small volumes (a few hundred per day) to your most engaged recipients first. Double the daily volume cautiously over four to six weeks while watching open and bounce rates. If you see complaints climbing, slow down. The goal is to teach the receivers' filters that your traffic is wanted.

Google Postmaster Tools and Microsoft SNDS expose what those two providers think of you. Both are free; both are essential the moment you start sending volume.

2. Content

SpamAssassin and similar engines weight a long list of heuristics. The most common offenders we see in WillItInbox reports:

• HTML-only emails with no plain-text alternative.
• Image-heavy messages with little real text. Image-only campaigns are immediate filter bait.
• Hidden text — display:none, white-on-white, zero-width characters. Spammers use these to evade text scanners; receivers know that.
• Trigger phrases stacked together. One "free" is fine. "FREE! Limited time! Act now!" is a stack and SpamAssassin scores it as one.
• All-caps subject lines and excessive punctuation.
• Suspicious links — shorteners, mismatched anchor text, HTTP-only URLs.

Individually each of these adds a small score. Together they compound, and you cross the spam threshold without ever touching anything obviously wrong.

3. List hygiene

High bounce rates and spam complaints destroy reputation faster than anything else. Even a single bad send can take weeks to recover from. The discipline is uncomfortable but non-negotiable:

• Validate addresses at signup with a real syntax + MX check.
• Honour unsubscribes immediately and irrevocably.
• Remove inactive subscribers after a quiet period (90 days is conservative; 180 is generous).
• Never reactivate old lists without a fresh re-permission campaign.

If you bought, scraped, or merged a list, it will hurt you. There is no exception to this rule.

What to fix first

Run a WillItInbox test and look at the Recommendations panel. Sort by severity — critical issues are almost always reputation or authentication, warnings are usually content. Fix the top three critical items, then re-test. You'll usually see a 10–20 point jump in score after one round.

If you only do one thing

Add a plain-text part to your HTML emails. It's the single most cost-effective content fix and it pleases SpamAssassin, accessibility tools, and old-school clients all at once.

Use the email deliverability tester, sample report, email validator, and domain monitoring together. No single score explains every spam-folder decision.

Apply the findings from Why your emails land in spam with the deliverability testing workflow, then use the relevant WillItInbox product workflow to collect current evidence.