Email on Acid Review

Email on Acid built its name on one job: show marketers exactly how an email will render before it goes to a real inbox, across dozens of clients and devices, so broken layouts get caught in QA instead of by an angry subscriber. That's still the core product, but the name itself is in transition — as of this writing, the vendor is actively migrating customers to a rebranded product called Mailgun Inspect, and the pricing and plan structure on the live site already reflect the new branding. This review covers what the tool does today, what it actually costs, and what we could and couldn't verify independently.

Verdict at a glance

What kind of tool is Email on Acid?

It's a pre-send email quality assurance platform, not an inbox provider or a sequencer. You build a campaign in whatever ESP or template tool you already use, then run it through Email on Acid to see how it actually renders in Gmail, Outlook, Apple Mail, and dozens of other clients and devices, catch spam-trigger content before it costs you inbox placement, and check whether the markup holds up for accessibility standards. It sits in the same category as Litmus, one layer removed from actual sending and deliverability infrastructure.

The vendor, Sinch/Mailgun, has folded the product into a broader QA suite under the Mailgun Inspect name, which the current marketing describes as combining pre-send checks, delivery, and post-send analysis into one platform built on Mailgun's existing sending infrastructure.

How it works

You connect a campaign — either by forwarding a test send to a unique address or through the API — and the platform renders it against 100+ real client and device combinations, typically returning results in under 10 seconds. From there, the higher tiers add spam and deliverability scoring split by B2B and B2C sending profiles, HTML code analysis, link and image validation, and accessibility checks against WCAG 2.2 and the European Accessibility Act. Every plan includes unlimited user seats and API access, so the workflow scales to teams without per-seat licensing friction.

Pricing (2026)

The real-cost catch is the overage rate: $0.15 per preview beyond your plan's allotment on both tiers. A team on Basic doing 1,000 previews a month is paying roughly $0.099 per preview already baked into the plan — go 200 previews over in a busy month and that's another $30, quietly pushing a $99 plan toward $130. Teams that test aggressively (multiple client/device combinations per campaign, multiple revision rounds) should model their actual monthly preview volume against the allotment before committing, not just compare sticker prices. Note too that this pricing and plan structure is published under current Mailgun Inspect branding — customers still on legacy Email on Acid contracts may see different numbers until their renewal date.

What we could and couldn't verify

We could verify current pricing, plan structure, and advertised features directly from the vendor's live site. We could not verify independent third-party sentiment: G2 and Trustpilot blocked automated access during this research, Capterra returned no listing under the product's current URL, and general web search turned up nothing usable within this research pass. That means any third-party rating line in most competitor reviews of this tool is either stale (referencing ratings tied to the old Email on Acid listing, which may not carry over cleanly to Mailgun Inspect) or unverifiable right now. We're not going to manufacture a rating or a sentiment summary we can't back with a source — treat any star rating you see elsewhere for this product with that caveat, and check G2/Trustpilot directly yourself before relying on it.

Pros and cons

Pros

• Broad client/device coverage (100+) with fast turnaround per preview
• Combines rendering, spam/deliverability, and accessibility checks in one workflow
• Unlimited seats and API access on every plan, no per-user licensing tax
• Real compliance certifications advertised (SOC 2 Type 1, ISO 27001/27701, GDPR)

Cons

• The product name and plan structure are actively changing (Mailgun Inspect migration), which is an unusual position for a tool being newly evaluated
• Overage pricing at $0.15/preview can add up quickly for high-volume QA teams
• No independently verifiable third-party review data found at time of writing
• Accessibility and spam-testing features are gated behind the $199 Premium tier, not included at the entry price

Alternatives

Email on Acid / Mailgun Inspect competes in pre-send email QA and rendering, not inbox infrastructure or sequencing, so this isn't a category where Slicey is a direct alternative:

• Litmus — the closest direct competitor, with a similar rendering-preview and analytics feature set and a longer independent track record.
• Mail-Tester — a free, lightweight spam-score checker for quick one-off sends rather than a full QA workflow.
• ZeroBounce — focused on list validation and deliverability scoring rather than rendering previews.
• Validity Everest (formerly Return Path) — enterprise-focused deliverability and inbox placement monitoring.
• Postmark / SendGrid built-in previews — basic rendering checks bundled into some ESPs, useful if you don't need dedicated QA depth.

Who should use it — and who shouldn't

Good fit: marketing teams and agencies sending frequent campaigns across many clients and devices, especially where accessibility compliance (WCAG, EAA) is a real requirement rather than a nice-to-have, and where broken rendering has caused real damage before.

Poor fit: small teams sending occasional campaigns who can get by with a free spam checker; anyone uncomfortable buying into a product mid-rebrand without clarity on how their specific plan and pricing carry forward; teams whose priority is cold-outreach inbox infrastructure or sequencing rather than pre-send creative QA — that's a different tool category entirely.