DevInbox (devinbox.io) is built for testing, and it is good at it: unlimited sandboxes, unlimited forwarders, unlimited webhooks, an MCP server, and a free tier that gives you 1,500 operations without a card. If you are writing integration tests that need to catch a signup email and assert on it, this is a sensible tool.
Bottom line: the two properties that make DevInbox a good test harness make it a poor production mailbox. Retention runs from one hour on the free plan to fifteen days on the $99 plan, so an agent cannot look back at a conversation from last month. And the meter counts operations — every received email, every forward, every webhook fired and every API call — so a polling agent burns quota without any mail arriving. Dead Simple Email keeps the mail, charges per email rather than per API call, and costs $29/mo where DevInbox's comparable tier is $99.
Pricing Comparison
Figures as read on 2026-09-16. DevInbox prices shown are its annual rates.
| Tier | Dead Simple Email | DevInbox |
|---|---|---|
| Free | 5 inboxes · 5,000 emails/mo · mail retained | 1,500 operations/mo · 1-hour retention |
| Entry paid | $5/mo — 15 inboxes, 15,000 emails | $12/mo — 10,000 operations, 24-hour retention |
| Mid | $29/mo — 100 inboxes, 100,000 emails | $39/mo — 25,000 operations, 7-day retention |
| Top published | $99/mo — 500 inboxes, 500,000 emails | $99/mo — 100,000 operations, 15-day retention |
| What the meter counts | Emails sent and received | Emails, forwards, webhooks fired and API calls |
| Message retention | Kept within your plan's storage | 1 hour to 15 days |
The operations meter is the part worth modelling before you buy. One inbound email that fires a webhook and is then fetched by your agent is three operations on DevInbox and one email on Dead Simple Email.
Feature Comparison
| Feature | Dead Simple Email | DevInbox |
|---|---|---|
| Message retention | Kept — 500 MB to 10 GB by plan | 1 hour to 15 days, then deleted |
| Billing unit | Emails | Operations, including API calls |
| Inbox count | 5 / 15 / 100 / 500 by plan | Unlimited sandboxes |
| Outbound sending | Yes — full send, reply, reply-all, forward | Forwarding-focused |
| Custom domains | Yes — plus domain purchase via API | Yes — unlimited from the $39 plan |
| IMAP / SMTP | Yes — every inbox | Not advertised |
| MCP server | Yes — in the official MCP registry | Yes |
| Webhooks | All plans, with delivery log and replay | Unlimited, metered as operations |
| Prompt-injection scanning | Yes — scored on every inbound | Not advertised |
| Verification-code / OTP extraction | Yes — dedicated endpoint | Message verification via API |
| Threading, drafts, templates, contacts | Yes | Not advertised |
| Agent identity / OIDC sign-in | Yes — Sign in with Dead Simple | No |
| SSO | Enterprise | $99 plan |
Retention Is the Real Difference
An agent that handles a support thread, a recruiting pipeline or a vendor negotiation needs to read what was said three weeks ago. Email is the only durable, portable memory most agents have — that is half the reason to give an agent an inbox at all. Fifteen days is the longest window DevInbox publishes, and one hour is what the free plan offers.
Dead Simple Email retains messages inside your plan's storage allowance, with threading, search and IMAP on top, so the inbox works as the agent's long-term record rather than a buffer.
Who Should Use Which
Use DevInbox for integration tests, signup-flow assertions, disposable sandboxes and CI — work where the mail is supposed to disappear and unlimited throwaway addresses are the point.
Use Dead Simple Email for anything an agent does in production: a mailbox it owns, keeps and can search, with IMAP, injection scanning, webhooks with replay, and 100,000 emails on the $29 plan.
Frequently Asked Questions
As of September 2026, one hour on the free Developer plan, 24 hours on the $12 Team plan, 7 days on the $39 Business plan and 15 days on the $99 Enterprise plan. After that the message is deleted. Dead Simple Email retains messages within your plan's storage allowance, so an agent can search a conversation from months ago.
Any email received or forwarded, any webhook triggered, and any call to the API. That means a single inbound message that fires a webhook and is then fetched by your agent consumes three operations. Dead Simple Email meters emails sent and received; API calls are free within your plan's rate limit.
As of September 2026 DevInbox is free for 1,500 operations, $12/mo for 10,000, $39/mo for 25,000 and $99/mo for 100,000, at annual rates. Dead Simple Email is free for 5 inboxes and 5,000 emails, $5/mo for 15 inboxes and 15,000 emails, $29/mo for 100 inboxes and 100,000 emails, and $99/mo for 500 inboxes and 500,000 emails.
It is built for testing, and the retention window shows it. For an agent that needs to remember a conversation past 15 days, or that must read mail through IMAP, or that needs inbound prompt-injection scanning, Dead Simple Email is the production-shaped option. For CI and integration tests, DevInbox is a good tool.
Dead Simple Email. Real inboxes that keep their mail, IMAP and SMTP on every one, HMAC-signed webhooks with delivery logs and replay, prompt-injection scanning on every inbound message, verification-code extraction, and 100 inboxes with 100,000 emails for $29/mo.
Yes, and it is a reasonable split: DevInbox for disposable test inboxes in CI, Dead Simple Email for the inboxes your agents actually own in production. Dead Simple Email's free plan also covers small test suites at 5 inboxes and 5,000 emails a month.
All DevInbox figures on this page were read from its public pricing page on 2026-09-16 and are reproduced for comparison. Vendors change prices; check the source before making a decision, and tell us if anything here is out of date. Full cross-vendor tables: AI agent email API pricing comparison.