Litmus vs Email on Acid: Is the Price Gap Worth It?
One is the established, feature-heavy option enterprise teams standardize on. The other now trades under a new name after an acquisition, and still undercuts it on price for the same core job: catching a broken render before a subscriber does.
Litmus is the deeper, more expensive tool: broader analytics, personalization, and collaboration features, sold through a largely sales-led pricing process. Email on Acid covers the same core job, previewing an email across dozens of clients and devices before you send, at a lower, self-serve price, but it no longer exists as a standalone brand: since a 2024 acquisition by Sinch, it now ships as Mailgun Inspect. If you need deep engagement analytics and dynamic personalization testing, pick Litmus. If you mainly need reliable client/device previews, spam testing, and accessibility checks without an enterprise price tag, Mailgun Inspect (the product formerly sold as Email on Acid) is the better fit.
Email testing and QA tools exist because "it looked fine in my inbox" tells you almost nothing about how an email renders everywhere else. Gmail, Outlook, Apple Mail, Yahoo, and dozens of other clients each use a different rendering engine, and several of them (Outlook on Windows especially) strip or mangle CSS that every other client handles fine. A layout that looks perfect in Apple Mail on an iPhone can collapse into a single unreadable column in Outlook on a desktop, and the person who built the email in their own inbox has no way to see that without a dedicated preview tool.
That's why these tools sit as a separate step from the ESP (email service provider) used to actually send the campaign. An ESP's job is building, sending, and tracking the email. A testing/QA tool's job is rendering that same email across a wide matrix of real clients and devices, running it through spam-filter checks, and increasingly, checking it against accessibility standards, before it ever reaches a subscriber. Skipping this step means finding rendering bugs the way most teams eventually do: from a customer complaint or a screenshot on social media, after the send already went out.
Feature-by-Feature Comparison
| Feature | Litmus | Email on Acid (now Mailgun Inspect) |
|---|---|---|
| Current ownership / branding | Owned by Validity, sold under the Litmus name | Acquired by Sinch (deal completed December 2024); now sold as Mailgun Inspect, with Email on Acid contracts transitioning over during 2026 |
| Client / device preview coverage | 100+ email clients and devices | 90-100+ email clients and devices, comparable coverage |
| Spam / deliverability testing | Included on core plans; deeper inbox-placement testing sits behind a separate Deliverability add-on | Content spam analysis included on paid plans without a separate add-on |
| Accessibility testing | Available as part of its QA checks | Built-in accessibility checks aligned to WCAG 2.2, positioned around the EU's European Accessibility Act (EAA) |
| Engagement analytics (opens, read time, geolocation) | A core strength: detailed analytics and engagement tracking | Limited to none: the product is built around pre-send previews and QA, not post-send engagement tracking |
| Personalization / dynamic content preview | Litmus Personalize: preview dynamic and rule-based content before send | Not a comparable feature |
| Collaboration / proofing tools | Built-in review and approval workflows for teams | Basic sharing of preview results; less built around multi-stakeholder proofing |
| API access | Available on higher tiers | API-first under Mailgun Inspect, aimed at embedding QA checks into other tools and workflows |
| Pricing model | Largely sales-led at most tiers; published self-serve pricing is limited | Self-serve monthly plans with published pricing and a free trial |
Pricing Compared
| Tier | Litmus | Email on Acid (Mailgun Inspect) |
|---|---|---|
| Entry | Reported to start around $79/month for a single user on legacy plans; current published pricing largely routes to a sales conversation | Basic, around $99/month for 1,000 previews/month, self-serve |
| Mid tier | A "Core" tier has been quoted around $500/month for roughly 2,000 previews/month and 5 full users | Premium, around $199/month for 2,000 previews/month, adds fuller spam and accessibility checks |
| Enterprise | Custom pricing: dedicated account management, custom preview/analytics volume, advanced permissions | Custom pricing through Mailgun Inspect for higher volume and white-label / embedded use cases |
| Overages | Typically negotiated as part of the contract | Roughly $0.15 per preview beyond your plan's monthly allotment |
Figures are approximate and gathered from current vendor and third-party pricing pages as of September 2026. Litmus in particular has moved toward sales-led, quote-based pricing at most tiers, so treat these numbers as a starting point for your own quote, not a guarantee.
Both vendors change plans, limits, and pricing periodically, and Email on Acid's transition to Mailgun Inspect is actively changing what's sold under which name through 2026. Confirm current plan names, preview limits, and pricing directly with each vendor before budgeting.
Pros & Cons
Litmus
Pros
- ✔ Deepest analytics and engagement tracking of the two, useful for teams that report on email performance beyond the ESP's own numbers
- ✔ Litmus Personalize lets you preview dynamic and rule-based content before it goes out, not just static layouts
- ✔ Strong built-in collaboration and proofing workflows for teams that route emails through multiple approvers
Cons
- ✕ Pricing is largely sales-led at most tiers, making it harder to budget without a quote
- ✕ The most expensive option of the two for comparable core preview functionality
- ✕ Deeper deliverability / inbox-placement testing sits behind a separate add-on rather than being included
Email on Acid (Mailgun Inspect)
Pros
- ✔ Comparable client and device preview coverage to Litmus at a meaningfully lower, self-serve price
- ✔ Spam testing and WCAG-aligned accessibility checks included without a separate add-on
- ✔ Published, self-serve pricing with a free trial, easier to evaluate without a sales call
Cons
- ✕ Much lighter on post-send engagement analytics than Litmus; it is built around pre-send QA, not tracking
- ✕ The Email on Acid brand itself is being phased out in favor of Mailgun Inspect through 2026, which can be confusing mid-transition for existing customers
- ✕ Overage fees per preview can add up for a high-volume team that regularly exceeds its plan's allotment
Which One Should You Actually Pick
You need engagement analytics beyond what your ESP already reports
Litmus is the stronger fit: read-time analysis, geolocation, and device-level engagement data are a core part of the product, not an add-on.
You send personalized or dynamically assembled content
Litmus Personalize previews rule-based and dynamic content before send, which Email on Acid / Mailgun Inspect does not offer a direct equivalent to.
You mainly need reliable client/device previews, spam testing, and accessibility checks on a budget
Mailgun Inspect (the product sold as Email on Acid) covers this core job at a fraction of Litmus's typical cost, with transparent self-serve pricing.
You are already an Email on Acid customer
Confirm your contract's transition timeline to Mailgun Inspect directly with Sinch. Plan names, limits, and login details are changing as part of the migration, and you don't want to find that out mid-campaign.
Why "It Looked Fine in My Inbox" Isn't a QA Process
Every marketer who has ever built an email has previewed it in their own inbox before sending, and that habit is exactly why rendering bugs keep shipping. A preview in one client, on one device, tells you how the email renders in that one specific combination of engine, screen size, and dark-mode setting. It tells you nothing about the other 90-plus combinations your actual subscriber list is opening the email in. A two-column layout that looks clean in Apple Mail on a laptop can stack incorrectly, hide an image, or break a call-to-action button entirely in Outlook on Windows, purely because Outlook renders HTML email through a different engine than nearly every other client. Neither Litmus nor Mailgun Inspect eliminates the need for a human to actually look at the results, but both replace the guesswork of "well, it looked fine to me" with an actual matrix of how the email renders everywhere it's likely to land.
A Rebrand Worth Knowing About Before You Buy
Anyone researching this comparison today is going to run into a naming problem: a lot of the content ranking for "Email on Acid" describes a product that, as a standalone brand, is being wound down. Sinch's acquisition of Email on Acid closed in December 2024, and by mid-2025 Sinch had launched Mailgun Inspect as the direct successor, built on Mailgun's own infrastructure and carrying forward Email on Acid's core testing capabilities along with additional accessibility features tied to the EU's European Accessibility Act. Existing Email on Acid contracts are being migrated over to Mailgun Inspect through 2026. Practically, that means a prospective buyer today should evaluate Mailgun Inspect directly rather than assume the legacy Email on Acid pricing, plan names, or sign-up flow are still current.
What Neither Tool Does for You
A clean pass across every client preview does not mean the email will perform well, and it doesn't mean it will avoid the spam folder either. Both tools test rendering and flag common spam triggers, but final inbox placement depends heavily on factors outside either product: domain authentication (SPF, DKIM, DMARC), sender reputation built up over time, and how your own list engages with past sends. A perfectly rendered email sent from a poorly authenticated domain, or to a list full of stale addresses, will still struggle to reach the inbox. Treat either tool as the pre-send QA step it's designed to be, not a substitute for the deliverability fundamentals that sit underneath every campaign regardless of which testing tool caught the layout bug first.
Common questions
Not really, going forward. Sinch acquired Email on Acid, with the deal completing in December 2024, and has since relaunched its capabilities under the name Mailgun Inspect, built on Mailgun's infrastructure. Existing Email on Acid contracts are transitioning to Mailgun Inspect during 2026, so a new buyer today is effectively evaluating Mailgun Inspect even if search results still reference the older Email on Acid name.
Usually, yes, if you send to a broad audience. Most ESPs only preview a small handful of clients, not the 90 to 100+ that Litmus and Mailgun Inspect each cover. Outlook's desktop rendering engine in particular behaves differently enough from web-based clients that an ESP's basic preview commonly misses layout breaks these dedicated tools catch.
No. Rendering correctly and avoiding the spam folder are two different problems. Spam and content testing inside these tools flags common red flags, but final inbox placement also depends on sender reputation, domain authentication (SPF, DKIM, DMARC), and recipient engagement history, none of which either tool controls directly.
Mailgun Inspect (Email on Acid) is generally the cheaper, easier-to-evaluate option for a small team, with published self-serve plans starting well below Litmus's typical entry cost. Litmus becomes more competitive on a per-feature basis only once you actually need its deeper analytics and personalization tools, which most small teams do not use fully.
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