A Mailosaur alternative for OTP testing in 2026

If you are searching for a Mailosaur alternative, you are almost certainly one of four people. You are an indie dev who hit the entry-tier pricing sooner than expected. You are a QA lead whose CI bill went sideways once the suite grew past a few hundred runs. You are a platform engineer who wants the same primitive without the Servers tab in the dashboard. Or you are an agent builder who needs an inbox that does not assume one human owner per workspace.

This post is a fair comparison, not a hatchet job. Mailosaur has been in this space longer than catchotp has existed and the company has earned a reputation for solid SDKs and a QA-friendly UI. We are going to be honest about what they do well, where the friction shows up, what catchotp does differently, and when each tool is the right pick. There is a side-by-side code example near the bottom, and a FAQ at the very bottom that mirrors the questions we hear most often during evaluation.

What Mailosaur does well

Mailosaur is the most feature-aligned peer in the receive-side email space. They have been shipping since 2014, they have a real SDK in seven or so languages, and they have the kind of QA-focused features that come from years of customer feedback.

The things that genuinely work:

• Server-and-inbox model that maps cleanly onto how QA teams already organize work. A “server” is a long-lived domain, and inboxes underneath it are addressable by any local-part on demand. Most tests do not need to call create_inbox at all.
• First-class screenshot-on-fail tooling and integrations with the big test frameworks. The Playwright and Cypress plugins are mature.
• HTML preview with a real rendering engine in the dashboard, which matters when you are debugging why a transactional email looks different in Outlook than in Gmail.
• An established compliance posture — SOC 2, GDPR, the audit-trail features that mid-market QA teams need to clear vendor review.

If you are a 200-person engineering org that already runs Mailosaur and the bill is acceptable, the honest advice is “do not migrate for sport.” Migration costs are real and Mailosaur is not the wrong tool for that shape of company.

Where Mailosaur friction shows up

Search trends for “Mailosaur alternative” are not idle. Six recurring frictions account for almost all of it.

1. Pricing tiers that bite at the wrong moment

Mailosaur’s entry tier sits at roughly £15 per month for an individual seat with limited messages. The next jump is into the Team range at £79 to £149-ish per month, depending on the line items. The friction is not the absolute price — it is that the jump happens right at the moment a working test suite starts to scale. You go from “this is cheap” to “this is a quarterly conversation” without an intermediate landing zone.

The catchotp position: a single $29 Pro tier covers individual devs and small teams, with usage-based overage rather than tier promotion when you grow.

2. The wait API has a polling shape

Mailosaur’s messages.get and messages.list accept a timeout parameter, but the client-side behavior is closer to “polling with a server-side budget” than to a true long-poll. In practice this means you set a 10-second timeout, the SDK polls until something matches or the budget runs out, and your test’s wall-clock time is bounded by how aggressive that polling is.

For OTP testing this is fine most of the time. It bites when (a) tests run at high concurrency and the polling-rate-limit kicks in, or (b) the email arrives 200ms after the request is fired and the test happens to wait 1.5 seconds for the next poll because of jitter.

3. Account-bound inboxes vs ephemeral

Mailosaur’s mental model is “you have N servers in your account and any number of inboxes under each.” This is great for organized QA teams. It is awkward for ephemeral CI workflows where you want a per-test inbox, scoped to a single test run, deleted on teardown, never reused. You can simulate ephemeral inboxes by using a unique local-part per test, but the inboxes-collection in the dashboard fills up with stale entries and the “delete after 24 hours” guarantee is not user-controllable.

4. Attachments handling

Mailosaur’s attachment API works but the path from “the email arrived” to “the PDF is on disk and parseable” runs through two SDK calls and a separate download. For the OTP-testing case attachments are usually irrelevant. For QA flows that involve invoice generation, contract delivery, or 2FA backup-code emails, the seam is more visible than it should be.

5. Regional availability

Mailosaur is UK-headquartered and the platform serves global traffic. There is no published per-region inbound, which means EU-resident customers are routing receive traffic through whichever region the platform happens to terminate in. For most teams this is a non-issue. For regulated industries with data-residency clauses in their DPA, it is a non-starter.

6. AI-agent ergonomics

Mailosaur predates the agent wave and the SDK shape shows it. There is no MCP server, no per-task scoped key model, no documented pattern for “an agent creates 50 inboxes in parallel and tears them all down.” You can absolutely make this work with the existing API, but the documentation does not meet you where you are.

What catchotp does differently

Three deliberate departures from the Mailosaur shape.

1. Three-verb API

The API surface is intentionally narrow: inboxes.create, inboxes.waitForOtp (and the more general messages.waitFor), and messages.get. There are not “servers” — every inbox is fully qualified on creation. There are not separate endpoints for “list all messages” and “wait for the next one”; the wait is the primitive, the list is a debugging convenience. The shape of the API matches the shape of what tests actually do.

import { CatchOTP } from '@catchotp/sdk';
const otp = new CatchOTP({ apiKey: process.env.CATCHOTP_KEY! });

const inbox = await otp.inboxes.create({ mode: 'ephemeral', ttlMinutes: 10 });
const code = await otp.inboxes.waitForOtp(inbox.id, { timeoutSeconds: 30 });

That is the integration. Two lines of setup, two lines of wait, no servers tab in the dashboard.

2. True long-poll waiter

The waitForOtp call holds an HTTP connection open server-side until the matching message arrives or the timeout fires. There is no client-side polling. p50 latency from email send to waiter resolution is around 600ms; p95 is under 1.2 seconds. We covered the EventBridge architecture that powers this in a separate post on the catchotp pipeline design — the short version is that long-poll on a Lambda Function URL backed by EventBridge rules per waiter is what makes the latency floor “as fast as the message arrives” rather than “as fast as the polling interval permits.”

This matters for two cases. CI tests that fan out to dozens of concurrent runs do not bunch up against polling rate limits. And tests where the email is delivered before the wait starts (yes, this happens — transactional providers are getting fast) resolve immediately rather than waiting for the next poll cycle.

3. Dual API + CLI + MCP server

The same operations are exposed three ways. The HTTP API and SDK are the primary interface. The catchotp CLI is a thin wrapper over the same API for shell-driven workflows — catchotp inbox create, catchotp wait <inbox-id>. The MCP server (@catchotp/mcp) exposes the inbox API as agent-callable tools, which means an LLM-driven agent can use catchotp from Claude Code, Cursor, or any MCP host with a one-block config. The AI-agent guide walks through a working agent example.

4. Pricing that does not promote you on growth

A single Pro tier at $29 per month covers individual devs and small teams. Usage above the tier is metered, not a forced promotion. The Team tier at $149 exists for org-wide quotas, audit log, and SSO, but it is genuinely the same product — not a feature-gated upsell.

The same OTP-wait, side by side

A realistic OTP-test scenario in both tools, written in TypeScript. Mailosaur SDK on the left (using the published mailosaur package), catchotp on the right.

Mailosaur

import MailosaurClient from 'mailosaur';

const mailosaur = new MailosaurClient(process.env.MAILOSAUR_API_KEY!);
const SERVER_ID = process.env.MAILOSAUR_SERVER_ID!;

async function getOtpForSignup(email: string): Promise<string> {
  // Trigger signup elsewhere; this code waits on the inbox.
  const message = await mailosaur.messages.get(SERVER_ID, {
    sentTo: email,
  }, {
    timeout: 30_000, // SDK polls server-side under the hood
  });

  // Mailosaur extracts codes via a regex match on the body.
  const codeMatch = (message.text?.body ?? '').match(/\b(\d{6})\b/);
  if (!codeMatch) {
    throw new Error('No OTP found in message');
  }
  return codeMatch[1];
}

catchotp

import { CatchOTP } from '@catchotp/sdk';

const otp = new CatchOTP({ apiKey: process.env.CATCHOTP_KEY! });

async function getOtpForSignup(): Promise<{ address: string; code: string }> {
  const inbox = await otp.inboxes.create({ mode: 'ephemeral', ttlMinutes: 10 });
  // Trigger signup with inbox.address elsewhere, then:
  const code = await otp.inboxes.waitForOtp(inbox.id, { timeoutSeconds: 30 });
  return { address: inbox.address, code };
}

Three differences worth pointing at. The catchotp call returns the address and the code from one helper, because creating an ephemeral inbox is a first-class operation rather than a “use a long local-part on the existing server” workaround. The waitForOtp returns the parsed code directly, with no client-side regex; the parser is the extraction pipeline we walked through covering the dozen common OTP formats. And the integration uses one secret (CATCHOTP_KEY) versus two (MAILOSAUR_API_KEY and MAILOSAUR_SERVER_ID), which sounds trivial until you are managing this in GitHub Actions across 30 repos.

When Mailosaur is still the right pick

Three scenarios where the honest advice is “stay on Mailosaur.”

If you are a regulated mid-market or enterprise team that already cleared Mailosaur through procurement, you have SOC 2 reports on file and a DPA negotiated. catchotp’s compliance posture is real but it does not yet cover every certification a Fortune 1000 vendor-management team will want. We are working toward parity, but you should not migrate just for our prices if the procurement work is already behind you.

If your test suite leans heavily on screenshot-on-fail comparison or HTML rendering checks across email clients, Mailosaur’s preview tooling is more mature than ours. catchotp’s dashboard renders the message and exposes raw HTML and text, but we do not ship a multi-client rendering matrix in 2026.

If your team is already three layers deep into Cypress + Mailosaur with custom commands wrapping their plugin, the migration cost is real. Net-new tests are easy; existing suite migration takes a sprint.

When catchotp is the right pick

The opposite pattern.

Solo developers and small teams who hit Mailosaur’s tier ceiling are the most common migration. The $29 Pro covers most use cases that bumped into the £79+ tier on the other side, and there is no “you have outgrown the entry tier” upsell.

AI-agent and MCP-driven workflows benefit from the catchotp shape directly. The MCP server is a one-block config in Claude Code or Cursor, the per-task key model maps onto agent isolation, and the long-poll latency matters more for agent loops than for human-driven tests.

OSS projects and one-off CI integrations get the most leverage from the ephemeral-inbox model. Create an inbox, run the test, let the TTL clean it up. The dashboard never accumulates cruft, the audit log captures what you need, and a free tier at 1,000 messages a month covers most repo-level test traffic without any payment commitment.

If you have never used Mailosaur, evaluating both is fair. The free tiers of each take an afternoon to compare. We provide a side-by-side comparison page and our pricing page is itemized for direct comparison.

FAQ

Is catchotp cheaper than Mailosaur?

For most usage patterns, yes — but the answer depends on volume. catchotp’s Pro tier is $29 per month with metered overage; Mailosaur’s individual tier starts around £15 per month and steps up to £79+ for team features. If you are below 1,000 messages per month and only need one seat, both are inexpensive. If you are at 10,000+ messages or need team features, catchotp typically lands at half the price or less, but you should run the math on your actual volume.

Does catchotp have an SDK like Mailosaur?

Yes. The TypeScript and Python SDKs are first-class. The shape is narrower than Mailosaur’s — three primary verbs versus a broader API surface — and the install is a single package (@catchotp/sdk or pip install catchotp). We also ship a CLI and an MCP server that share the same authentication and operations.

Can I migrate from Mailosaur to catchotp?

Most migrations are two files and an afternoon. Replace the Mailosaur client with the catchotp client, swap mailosaur.messages.get for otp.inboxes.waitForOtp, drop the regex extraction (catchotp parses the OTP for you), and update your CI secrets. The test logic does not change — you are swapping the receive-side primitive, not rewriting the assertions. The longest part of the migration is usually waiting for procurement to add the new vendor.

What about Mailosaur’s screenshot-on-fail feature?

catchotp does not currently ship a multi-client rendering matrix. If that feature is core to your testing workflow, this is a gap. We render the message HTML and text in the dashboard for debugging, and the raw MIME is downloadable, but we do not show “what this looks like in Outlook 2019 vs Gmail web.” If you need that, Mailosaur and a couple of dedicated email-rendering services like Litmus or Email on Acid cover the use case better.

Does catchotp support custom domains?

Yes, on the Team tier. You point an MX record at our inbound, we manage DKIM signing for you, and your tests can run against *@inbox.yourdomain.com. This is the same shape Mailosaur offers under their custom-domain feature. On the individual Pro tier, you use addresses on @inbox.catchotp.com — sufficient for almost all testing use cases.

How does catchotp handle attachments?

Attachments come back in the parsed message as a list with metadata (filename, content-type, size) and a presigned URL for the body. There is no separate download endpoint to call — the URL is in the message JSON, valid for 10 minutes, and you fetch it directly. For PDFs and images this is one less round-trip than the equivalent Mailosaur path.

What happens if my test runs faster than the email arrives?

Both tools handle this correctly. catchotp’s waitForOtp is a true long-poll and will block until the matching message arrives or the timeout fires; the message can arrive before, during, or after the wait starts. Mailosaur’s behavior is similar but client-side polling means you may pay a small latency penalty if the email arrives in the first 200ms of the wait. For OTP testing the practical difference is usually 0.5 to 1 second per test.

Can I run catchotp from my CI without a credit card?

Yes. The free tier covers 1,000 messages per month and 5 concurrent inboxes — enough for a small CI pipeline running a few hundred email-using tests per month. You upgrade when you hit the parallelism cap (more than 5 concurrent inboxes) or when you need longer retention than the free 24 hours.

Try the alternative. Free tier with 5 inboxes and 1,000 messages a month, no credit card. Start free or read our pricing breakdown for the Pro and Team details.

Related reading

• Programmable Email vs Disposable Email — the category framing that decides which tools are even in the comparison.
• How to Test OTP Flows in 2026 — the practical end-to-end test patterns.
• The OTP testing use case covers the integration story for QA teams in more detail.

The shorter version: Mailosaur is a respectable peer with a different shape. catchotp is faster on the wait, narrower on the API, friendlier on the price, and built for both human-driven tests and agent-driven workflows. If you are evaluating, run both free tiers side by side for a week. The right tool will be obvious by the end.