Flowproof and email-testing inboxes
You built an app, maybe by prompting an AI, and it sends email: sign-up links, password resets, receipts. Do you need an inbox API like Mailsac, Mailosaur, Testmail or MailSlurp, or do you need Flowproof?
The short answer
An inbox API gives you a place for test email to land and a way to read it from code. You still decide which emails to test, write the test that triggers each one, judge what arrived, and delete the fake users your tests created. That is the right tool if you already have a test suite and just need a fast inbox.
Flowproof does the other parts. Your AI finds every email your app sends, signs up with throwaway users, grades what arrives, and removes the test users afterwards. If you have no test suite and are not sure what to test next, that is the gap it fills.
If you already have a suite and need a fast inbox, the others are great and cheaper at volume. They also have things Flowproof does not: high-volume CI testing, uptime guarantees, and single sign-on for larger teams.
What each one does for you
| Tool | Gives you test inboxes | Finds every email your app sends | Grades what arrives | Deletes the test users from your app | Works from an AI chat with no code | Starting price |
|---|---|---|---|---|---|---|
| Flowproof | Yes | Yes, an email map from your code or plan | Yes: links, single-use tokens, SPF, DKIM and DMARC, merge tags, plain-text part, stock templates | Yes, with an itemized receipt | Yes, in claude.ai and ChatGPT | Free, no card. Flow Pass $29 once for 30 days |
| Mailsac | Yes | Not listed | Not listed | Not listed | Has an official MCP server for AI coding agents, which run code for you | Free: 1,500 operations a month. Paid from $18 a month |
| Mailosaur | Yes | Not listed | Content, layout and link checks, in its own words | Not listed | Not listed | From $20 a month billed yearly, 14-day free trial |
| Testmail | Yes | Not listed | A SpamAssassin spam report on each email | Not listed | Not listed | Free: 100 emails a month. Paid from $9 a month billed yearly ($12 monthly) |
| MailSlurp | Yes | Not listed | Content audit and inbox placement checks, in its own words | Not listed | Says agents can get accounts with scoped API and MCP access | Free: 500 emails received a month. Paid from $19.99 a month |
“Not listed” means we did not find it on that vendor’s pricing or home page when we checked. It does not prove the product can’t do it, so check their docs if it matters to you. Prices are in US dollars and change; the links below go to each vendor’s own pages.
Which should you choose
Choose Flowproof if…
- You are not sure which emails your app even sends.
- You have no test suite, or you built the app with an AI and do not want to write one.
- You want to be told what is wrong with each email, worst first, in plain words.
- You do not want test accounts left behind in your real database.
- You are still building and want the emails planned before they exist.
Choose an inbox API if…
- You already have Cypress, Playwright or similar tests that need an inbox to read.
- You run a lot of tests in CI, where price per email matters.
- Your company needs an uptime guarantee or single sign-on. Testmail lists a 99.99% uptime SLA and SAML or OIDC sign-on on its higher plans.
- You also need SMS testing or email previews on real devices (Mailosaur lists both).
- You are happy to write the tests and judge the results yourself.
You can also use both: an inbox API inside your own tests, and Flowproof to find the emails you forgot to test.
Cleanup is included on every plan, including free. See the plans in full.
Sources
Checked October 2026. Each fact above comes from the vendor’s own pages:
- Mailsac: Pricing, MCP server
- Mailosaur: Pricing, Home page
- Testmail: Pricing, Home page
- MailSlurp: Pricing, Home page
Something out of date or wrong? Email support@flowproof.dev and we will fix it.