Deliverability physics (what filters score)

• Authentication first. SPF, DKIM, and DMARC alignment are now table stakes for bulk senders.¹
  
  
• Engagement‑weighted filtering. Sustained opens, clicks, and low complaint rates improve reputation; CTOR is more reliable than begins in the Apple MPP era.⁸ ¹³
  
  
• Semantic robustness. Filters like RETVec are resilient to character tricks; they score the content. Bloated HTML, image‑only emails, and link farms are risk multipliers.³ ⁵
  
  

So, when does plain text win—and by how much?

• Triggered, one‑to‑one moments (trial activations, onboarding nudges, renewal notices): plain‑text or “hybrid‑minimal” emails typically produce double‑digit open lifts and meaningful CTOR gains, with documented cases of ~20% → ~60% opens after simplifying templates.⁴ ⁵
  
  
• Outreach and lifecycle messages (hand‑off from SDRs, success managers): plain text reduces complaint risk and often improves replies, especially with human closings and a single, specific ask.⁶ ⁸
  
  
• Newsletters and promotions: keep HTML, but strip to essentials; add BIMI for trust. Expect 10%+ open improvements where logo display is active and authentication is enforced.⁹ ¹⁰ ¹¹
  
  

Implementation playbook (30–60 days)

Week 1–2 — Compliance & telemetry.
Authenticate (SPF, DKIM), enforce DMARC, register Postmaster Tools, and confirm complaint rates <0.3%. Tag sends so you can compare HTML vs. plain‑text cohorts at the segment level.¹ ²

Week 3–4 — Plain‑text pilots.
Convert three journey emails (welcome, first‑value, reactivation) to plain text with one link, short preheaders, and human sign‑offs. Ship as multipart/alternative so clients that prefer HTML still render. Keep tracking links to a minimum.

Week 5–8 — Measure like a CFO.
Report inbox placement (not just delivery), CTOR, complaint rate, and downstream conversions. Hold design “wins” to the same standard: outweigh the heavier code footprint with CTR or revenue per send. Use BIMI for branded sends once DMARC is enforced.⁹ ¹

Guardrails.
Prompts from AI copy tools are fine—but the best performers read like a note you wrote. Avoid image‑only layouts; if you need visuals, keep weight low and links clean.³ ⁵

What the research can—and can’t—tell you

• Strong practitioner evidence. Most modern data comes from vendor case studies, deliverability platforms, and A/B tests (e.g., FROGED, HubSpot‑reported experiments, Mailtrap summaries). Take sample sizes and verticals into account.⁴ ⁵ ⁶
  
  
• Authoritative rules on sender reputation. The Gmail/Yahoo bulk‑sender policies and Google’s own RETVec documentation are primary sources for the “physics” of reach.¹ ² ³
  
  
• Limited peer‑reviewed work comparing layout types head‑to‑head in the post‑MPP era. That’s why CTOR and complaint rates—not raw opens—are your proving ground.⁷ ⁸ ¹³
  
  

How Geeks for Growth can help

• Deliverability Audit (Gmail/Yahoo compliance). We fix authentication, alignment, and feedback loops; stabilize complaint rates under 0.1% to create reputation headroom.
  
  
• Plain‑Text Copy Lab. Two‑day sprints that rewrite your highest‑value journeys into minimalist, human‑tone variants.
  
  
• BIMI & DMARC Enablement. From policy design to VMC issuance and logo testing across major mailbox providers.
  
  
• Experimentation & Reporting. Lift‑tested rollouts with Postmaster monitoring, CTOR dashboards, and revenue attribution that a CFO will sign off on.
  
  

Bottom line: In 2025, filters reward emails that look like the messages people send each other. Use plain‑text (or near‑plain) for moments that should feel personal, reserve design for campaigns that truly need it, and wrap everything in the authentication and reputation discipline the inbox now demands.

Endnotes

1. Google Workspace Admin Help, “Email sender requirements and guidelines” (SPF, DKIM, DMARC; unsubscribe requirements). (Google Help)
   
   
2. Google Workspace Admin Help, “Bulk sender guidelines FAQ” (0.3% spam‑rate mitigation threshold). (Google Help)
   
   
3. Google Online Security Blog, “Improving Text Classification Resilience and Efficiency with RETVec” (Gmail spam filter upgrade). (Google Online Security Blog)
   
   
4. MarketingSherpa, “Software platform switches to plain text emails, increases open rate to 60%” (FROGED case). (MarketingSherpa)
   
   
5. Mailtrap, “HTML vs. Plain‑Text Email,” summarizing HubSpot testing; added images/GIFs, cut opens 23%–37% on identical copy. (Mailtrap)
   
   
6. HubSpot, “Plain‑Text vs. HTML Emails: Which Is Better?” (multiple tests favor simpler/plain‑text). (HubSpot Blog)
   
   
7. Twilio SendGrid, “Email Marketing Benchmark Report” (baseline click‑rate context). (Twilio)
   
   
8. Twilio SendGrid, “Click‑to‑Open Rate: How relevant is my email?” (CTOR as a core effectiveness metric). (Twilio)
   
   
9. Red Sift, “What is BIMI?” and related research (open‑rate lift to ~39% when logo displays; trust effects). (Red Sift)
   
   
10. Valimail, “What is BIMI?” (average ~10% open‑rate increase; DMARC enforcement prerequisite). (Valimail –)
    
    
11. Oracle/Verizon references via Oracle Marketing Cloud blog (10% average uplift; studies citing 21%+). (Oracle Blogs)
    
    
12. Mailjet, “Measuring inbox placement” (Promotions tab is part of the inbox; not a penalty). (Mailjet)
    
    
13. Dyspatch, “Email marketing statistics 2025” (MPP distorts opens; industry CTOR context). (Dyspatch)