Deliverability After Gmail AI: Technical Best Practices for Email Attachments and Images

Hook: Why Gmail’s AI Should Change Your Attachment Strategy Right Now

If you send attachments or image-heavy campaigns to Gmail users, you’re facing a new reality in 2026: Gmail’s Gemini-powered AI is changing how content is rendered, summarized, and judged for safety. That matters because these AI-driven signals now feed into inbox placement and conversion paths—so broken images, unsafe attachments, or a misinterpreted subject line can kill both deliverability and revenue.

This article distills late-2025 and early-2026 developments and gives you a technical playbook to keep emails delivered, secure, and converting—covering image rendering, attachment handling, AMP email, spam signals, QA, and compliance-focused file workflows.

The evolution in 2026: How Gmail AI changed the inbox

In late 2025 Gmail rolled more Gemini 3 features into the inbox—AI Overviews, smart summaries, and deeper content analysis that go beyond keyword matching. By early 2026 these tools are widely available to Gmail’s ~3 billion users. The practical effects for senders are:

• Automated summaries can hide CTAs if key text lives only inside images or PDFs.
• Image proxying and caching are stricter—Gmail rewrites and hosts remote images, which alters cache behavior and tracking.
• Attachment sandboxing and file-type filtering are more aggressive; suspicious attachments are flagged or opened in secure viewers.
• AI-driven spam and safety signals now include semantic quality checks (AI slop detection), personalized risk scoring, and metadata analysis.

Why this is more than a UX change

These behaviors change the signal pipeline Gmail uses to decide inbox placement. Open tracking becomes noisier when images are proxied; engagement signals shift toward replies and time spent reading; and attachments that force extra friction reduce conversions. Your technical fixes must therefore address rendering, authentication, and user-experience fallbacks.

Top deliverability problems introduced or amplified by Gmail AI

1. Hidden CTA risk — AI summaries often omit image-only CTAs, buttons embedded in PDFs, or content behind scripts. If the AI overview doesn't surface a clear value prop, users may not open the message.
2. Image caching & tracking mismatch — Gmail’s image proxy rewrites URLs and caches aggressively. Open-rate signals drop and stale images may appear unless you design for proxy behavior.
3. Attachment quarantine & conversion friction — Attachments with uncommon extensions, nested ZIPs, or encrypted PDFs are more likely to be sandboxed, quarantined, or trigger phishing warnings.
4. AI slop penalties — Low-quality or obviously AI-generated copy (the 2025 “slop” trend) lowers engagement and may be weighted as lower-quality content by Gmail’s models.
5. MIME and encoding problems — Broken multipart/related or wrong Content-Type headers cause rendering failures in Gmail’s updated renderer.

Actionable technical fixes: Image rendering and tracking

Start with the principle: treat the proxied image as an opaque object and design fallbacks around it. Below are exact steps to fix rendering and retain reliable engagement signals.

1. Serve images with robust headers and predictable URLs

• Use HTTPS with a strong TLS config (TLS 1.2+; prefer 1.3). Gmail will only proxy secure images.
• Include correct Content-Type headers (image/jpeg, image/png, image/webp, image/avif). Gmail’s renderer favors explicit types.
• Avoid long-lived cache-busting querystrings. Instead, use per-send tokens if the image must be unique (e.g., /img/offer-sendid.jpg), but limit token variety to protect host reputation.

2. Always include text fallback and top-line value

Gmail AI uses text to summarize messages. Ensure your most critical message—offer, CTA, next step—exists as plain text in the top 120 characters and as part of the visible preheader. This guards against summaries that omit image content.

3. Use srcset + width/height to avoid layout shifts

Provide multiple resolutions via srcset and explicit width/height attributes so Gmail’s renderer can choose the right file and avoid reflows. Use WebP or AVIF for efficiency but keep JPEG/PNG fallback for older viewers.

4. Adjust tracking approach: favor aggregate events and server-side signals

• Expect image opens to be undercounted. Rely more on clicks, conversions, reply rates, and time-on-message (where available) in your deliverability metrics; add server-side observability for reliable signals.
• Shift to server-side event tracking (webhooks, postbacks) when link clicks lead to a landing page that records UTM-tagged visits.

Actionable technical fixes: Attachments and file handling

Attachments are now a dual problem: deliverability risk and compliance risk. Use these best practices to reduce friction and keep content accessible.

1. Prefer secure hosted files + signed short-lived URLs over raw attachments

Google increasingly routes attachments through viewers and Drive conversions. Host large files in a secure cloud bucket and include a short-lived signed URL in the email body. Benefits:

• Less chance of sandboxing or quarantine compared with raw attachment blocks.
• Better analytics (server logs, bot filtering) and the ability to require authentication on sensitive documents.
• Easier deletion and retention control for compliance (GDPR/CCPA).

Example signed URL pattern (conceptual):

<https://cdn.example.com/docs/offer-123.pdf?sig=abc123&exp=1700000000>

For guidance on short-lived links and storage governance, see the Zero‑Trust Storage Playbook.

2. If you must attach files, use standard MIME types and avoid nested archives

• Use application/pdf for documents; include PDF/A for long-term archiving if compliance requires it.
• Avoid sending .exe, .scr, or nested ZIP files. If you need compression, use .zip but avoid executables inside.
• Ensure Content-Disposition is explicit (inline or attachment) and matches intent. Example: Content-Disposition: attachment; filename="report.pdf".

3. Avoid password-protected attachments unless absolutely necessary

Password-protected files raise red flags for spam filters and AI safety modules. If you need to protect PII, prefer authenticated download flows or encrypted storage with short-lived links.

4. Mark attachments for scanning and retain scans as metadata

Run attachments through anti-malware engines and attach scan metadata to message headers or your server logs. If Gmail’s AI queries your server for provenance, you’ll have evidence of proper handling.

AMP email: opportunities and new caveats in 2026

AMP offers interactive messaging that can reduce the need for attachments. Gmail still supports AMP, but the onboarding and security posture are stricter than ever.

• Register for AMP with Google and maintain DKIM/DMARC alignment. AMP requires correctly signed messages with the text/x-amp-html MIME part plus a fallback HTML part.
• Provide a robust HTML fallback. Gmail AI may generate previews from the fallback HTML rather than the AMP content.
• Keep AMP permissions minimal: avoid remote JS, and don’t rely on third-party cookies—AMP components are sandboxed.

Authentication, reputation, and AI-aware signals

Authentication is table stakes. Gmail’s models combine traditional auth with new behavioral signals; get both right.

Authentication checklist

• SPF: Keep the sending IPs updated; use include mechanisms sparingly.
• DKIM: Use a 2048-bit key, and rotate regularly. Ensure DKIM signs the correct header set (From, Subject, MIME parts).
• DMARC: Enforce a p=quarantine or p=reject policy once comfortable. Gmail’s BIMI and brand signals prefer DMARC reject.
• ARC: Implement ARC for legitimate forwarded messages to preserve authentication reputation across relays.
• BIMI: If you have a strong brand domain and DMARC reject, publish a BIMI record and SVG Tiny P/SVG logo for brand trust cues in supported inboxes.

Reputation & engagement

Focus on real engagement: replies, clicks, and conversions matter more than opens in the AI era. Use behavioral suppression (suppress unengaged users) to keep complaint rates low and engagement high.

QA and testing: practical steps email QA teams must add in 2026

Add these tests to your pre-send checklist. These are developer-facing and actionable.

1. Header & Mime validation — Use tools (OpenDKIM, DKIMVerify, or your sending provider’s diagnostics) to confirm DKIM, SPF, and DMARC and validate MIME structure (multipart/alternative and multipart/related correctness).
2. Attachment checksum & virus scan — Scan every attachment and log hashes; fail sends when a file fails the scan.
3. Image rendering QA — Test renders across Gmail’s web, mobile, and Gemma-powered views (where available) using tools like Litmus or Email on Acid; use edge-first image QA to validate srcset and sizing.
4. Inbox placement testing — Seed to multiple Gmail accounts of varying ages and to Google Postmaster Tools and track placement and spam classification across sends.
5. Summary verification — For high-value sends, validate what Gmail’s AI summary shows by sending to a test Gmail account and documenting the AI Overview content. If the summary omits CTAs, restructure the email.

Privacy, compliance, and secure temporary file handling

With Gmail’s AI possibly exposing attachments to Google viewers and Drive conversions, privacy controls are crucial. Build file workflows that are auditable and ephemeral.

• Short-lived storage: If you host files, set default retention of 24–72 hours unless user consent or regulatory needs demand longer retention.
• Access control: Require authentication for PII or sensitive documents; use JWT-backed short links and IP gating for corporate distributions.
• Encryption: Encrypt files at rest and in transit. Log access events to support breach notifications and audits.
• Consent & notices: Include clear privacy language in the email body when sending attachments that contain PII and offer an easy way for recipients to request deletion.

Advanced strategies: automation, APIs, and developer workflows

To scale safely and stay adaptive, integrate conversion and deliverability checks into CI/CD for your email campaigns.

• Pre-send hooks: Add an automated pre-send pipeline that validates headers, runs anti-malware checks, ensures proper MIME types, and verifies that the top-line text meets the summary-first rule. See guidance on hardening local JavaScript tooling for secure developer hooks.
• Send-time personalization: Use server-time personalization so critical text is inlined as plain text (not just images). This helps AI summaries and personalization signals.
• API-based attachment delivery: Decouple large file delivery from SMTP by sending links tied to API tokens. Revoke tokens on unsubscribe or complaint events automatically.

Case study (anonymized): Recovering inbox placement with AI-aware fixes

A mid-size publisher saw Gmail placement fall 12% after rolling a PDF-heavy weekly report in Q4 2025. The root causes were image-only lead messages, password-protected ZIPs, and stale DKIM keys.

Remediation steps implemented over four weeks:

1. Moved large PDFs to authenticated short-lived links and removed password protection.
2. Ensured the top 90 characters contained the offer summary and a plaintext CTA.
3. Rotated DKIM keys and tightened DMARC to p=quarantine while monitoring via Google Postmaster Tools.
4. Suppressed long-term unengaged users and launched a re-engagement flow with explicit consent.

Result: Gmail inbox placement rebounded by 14 percentage points within six weeks and conversion rate from Gmail traffic increased by 18%—largely because AI Overviews began surfacing the plaintext CTA and the secure links reduced quarantine events.

Predictive look: What to expect from Gmail AI through 2027

Expect Gmail’s AI to increase its reliance on semantic quality metrics and behavior-based signals—replies, time-on-mail, and explicit CTA clicks—over raw opens. Images will remain proxied, but Google may introduce tighter heuristics for dynamic content and ephemeral links, especially for security-sensitive file types.

Actionable prediction: treat Gmail as a text-first, engagement-driven channel where attachments are a last resort and interactive in-email experiences (AMP or authenticated microapps) replace bulky files when compliance allows.

Email QA checklist: Quick run-before-send items (developer-friendly)

1. Validate SPF, DKIM (2048-bit), and DMARC alignment.
2. Confirm multipart structure: plain text first, HTML alternative, AMP part optional.
3. Ensure top-line plaintext contains the headline and CTA (first 120 chars).
4. Host images on HTTPS; set explicit Content-Type; test srcset variations.
5. Prefer signed short-lived URLs for sensitive files; avoid password-protected attachments.
6. Run virus scans and attach scan metadata to logs.
7. Seed tests to Gmail test accounts; validate AI Overview and attachment behavior.
8. Review copy for AI slop; add human QA to preserve structure and voice.

Final takeaways: Make Gmail AI work for your deliverability

Gmail’s AI is not a blocker—it's a new evaluator. Optimize for text-first clarity, secure hosted files, and behavioral engagement to win inbox placement in 2026.

• Design for summaries: Put the most important CTA and offer in plaintext at the top.
• Host files securely: Use signed short-lived links rather than risky attachments.
• Keep authentication tight: Strong SPF/DKIM/DMARC and ARC preserve reputation.
• Update QA: Test AI Overviews and attachment handling before every major send.
• Prioritize privacy: Encrypt, log, and expire files to meet compliance demands.

Next steps — action plan you can implement this week

1. Audit your last 10 Gmail sends for: top-120-char CTAs, hosted vs attached files, and DKIM freshness.
2. Replace password-protected attachments with authenticated short-lived links where possible.
3. Add an automated pre-send check to validate MIME structure and attachment scan status.
4. Seed each campaign to at least three Gmail test accounts and document the AI Overview summary.

Call-to-action

Need a fast deliverability and attachment security audit tailored to Gmail’s Gemini-era rules? Book a 30-minute technical review with our deliverability team at Converto.Pro. We’ll run your last three sends through our AI-aware QA toolchain, return a prioritized fix list, and help you set up automated pre-send checks to protect inbox placement and compliance.

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