Mails.ai is one of the sharper products in this category on the security side. It scores every inbound message for prompt injection across six categories, attaches sender reputation, blocks cold email at the API level rather than in a policy document, and offers optional intent and entity classification. The free tier needs no card, the SDKs are TypeScript and Python, and there is an MCP server.
Bottom line: the marketing says "no per-inbox pricing" and the pricing page says the $20 Pro plan allows a maximum of five agents. That is a per-inbox limit with a different name, and it is a tight one: an eleven-agent fleet pays $99/mo on Mails.ai and $5/mo on Dead Simple Email. Dead Simple Email also scans every inbound message for injection, and publishes what each plan includes without a cap hiding one tier down.
Pricing Comparison
Figures as read on 2026-09-16.
| Tier | Dead Simple Email | Mails.ai |
|---|---|---|
| Free | 5 inboxes · 5,000 emails/mo | 1 agent · 3,000 events/mo |
| Entry paid | $5/mo — 15 inboxes, 15,000 emails | $20/mo — 5 agents, 50,000 sends + 50,000 inbound |
| Mid | $29/mo — 100 inboxes, 100,000 emails | None — next tier is $99 |
| Top published | $99/mo — 500 inboxes, 500,000 emails | $99/mo — unlimited agents, 250,000 sends + 500,000 inbound |
| Cost for 6 agents | $5 | $99 |
| Cost for 15 agents | $5 | $99 |
| Cost for 100 agents | $29 | $99 |
| Metered option | $0.40/inbox, $0.20 per 1,000 emails over plan | Announced: $0.001 per send, $0.002 per inbound |
Above 100 agents Mails.ai's unlimited-agent Scale plan is a genuinely good deal on agent count — but its 250,000 sends are half of what Dead Simple Email's identically priced plan allows, and the jump from 5 agents to unlimited happens in one $79 step with nothing in between.
Feature Comparison
| Feature | Dead Simple Email | Mails.ai |
|---|---|---|
| Prompt-injection scanning | Yes — scored on every inbound, all plans | Yes — six categories, with sender reputation |
| Intent / entity classification | No | Yes — $0.003 per inbound on metered billing |
| Cold-email blocking enforced in the API | Yes — acceptable-use enforcement and abuse guard | Yes |
| Agents / inboxes on the entry plan | 15 for $5 | 5 for $20 |
| Webhooks | All plans, with delivery log and replay | Not advertised |
| IMAP / SMTP | Yes — every inbox | Not advertised |
| SSE / WebSocket streaming | Yes — both | Not advertised |
| Custom domains | Yes — paid plans, plus domain purchase via API | Pro and above |
| Visual dashboard | Yes — all plans | Not advertised |
| MCP server | Yes — in the official MCP registry | Yes |
| SDKs | Python, TypeScript, Go, CLI | TypeScript, Python |
| Verification-code / OTP extraction | Yes — dedicated endpoint | Not advertised |
| Agent identity / OIDC sign-in | Yes — Sign in with Dead Simple | No |
| Pay per request without an API key | Yes — x402 USDC and Stripe MPP | Metered billing announced, not live |
Where Mails.ai Wins
- Unlimited agents at $99. If you are running thousands of agent identities with modest traffic each, that ceiling is higher than anyone else's published one. Check the send allowance against your volume first.
- Classification on inbound. Optional intent and entity extraction on every message is a real feature, and it is priced separately and fairly at $0.003 per inbound.
- Sender reputation on the wire. Getting a reputation score attached to each inbound message is a nice primitive for deciding what an agent should act on.
Who Should Use Dead Simple Email
- Fleets of 6 to 100 agents, which is the range Mails.ai's ladder skips entirely.
- Anything event-driven. Webhooks with a delivery log and replay, plus SSE and WebSocket, on every plan.
- Anyone who needs IMAP for human review, migration or existing tooling.
- Send-heavy workloads, where 500,000 emails at $99 beats 250,000.
Frequently Asked Questions
Not quite. As of September 2026 the free plan allows 1 agent and the $20 Pro plan allows a maximum of 5 agents; only the $99 Scale plan is unlimited. So there is an agent cap, it is just expressed as a plan limit rather than a per-unit price. A six-agent fleet pays $99/mo on Mails.ai and $5/mo on Dead Simple Email.
As of September 2026 Mails.ai is free for 1 agent and 3,000 events, $20/mo for 5 agents with 50,000 sends and 50,000 inbound, and $99/mo for unlimited agents with 250,000 sends. Dead Simple Email is free for 5 inboxes and 5,000 emails, $5/mo for 15 inboxes, $29/mo for 100 inboxes and 100,000 emails, and $99/mo for 500 inboxes and 500,000 emails.
Yes, on every plan including free. Each inbound message is scanned and scored before your agent reads it, so an agent does not act on instructions embedded in mail from a stranger. Mails.ai does the same across six categories and adds a sender reputation score.
Neither is an advertised feature as of September 2026. Dead Simple Email provides HMAC-signed webhooks with a delivery log and one-click replay on every plan, plus SSE, WebSocket, and IMAP on port 993 and SMTP on port 587 for every inbox.
Dead Simple Email. It matches Mails.ai on inbound injection scanning and cold-email enforcement, and covers the 6-to-100 agent range that Mails.ai's pricing skips: 15 inboxes for $5/mo and 100 for $29/mo, against Mails.ai's 5 agents for $20 and unlimited for $99.
Above a few hundred agents with light traffic each, where unlimited agents for $99 is hard to beat, and when you want intent and entity classification on inbound messages as a built-in rather than something you run yourself.
All Mails.ai 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.