Block throwaway emails at signup
Find out if an email address is disposable, temporary or an automated inbox, including new domains that public blocklists don't know yet.
5 free checks a day, no signup needed.
Use it in your signup flow
Call the API when someone signs up, then decide what to do with risky addresses: block them, ask for phone verification, or send them to review.
curl -H "Mailsac-Key: YOUR_API_KEY" \ https://mailsac.com/api/signup-risk/new.user@example.com
{
"email": "new.user@example.com",
"score": 70,
"verdict": "likely_fake",
"category": "agent_or_test_inbox",
"signals": [
{ "id": "programmatic_inbox", "weight": 55,
"label": "Mail for this domain is received by Mailsac, an inbox service used by AI agents and test automation" }
]
}What it checks
Disposable services
Matches the domain against more than 100,000 known disposable email domains.
Agent and test inboxes
Flags addresses at, or domains whose mail is received by, inbox services built for AI agents and test automation, such as Mailsac, AgentMail, Mailosaur and MailSlurp.
Mail servers
Looks up where the domain's mail actually goes, so a brand-new domain pointed at a throwaway service is caught even when no list has it yet.
Domain age
Newly registered domains are a common sign of account farming.
Address shape
Machine-generated names, test and bot names, and +tags used to create many accounts from one inbox.
Questions
- How accurate is it?
- It is a risk score, not proof. It combines public signals such as disposable lists, mail servers, domain age and the shape of the address. Use it to add friction or review, for example asking for phone verification, rather than as the only reason to reject someone.
- Can it tell if an AI agent signed up?
- It flags addresses that belong to inbox services AI agents and test automation use, and domains set up the same way. An agent using a normal personal mailbox looks like a person, so combine the score with your other signup checks.
- Is it free?
- You can run a few checks a day on this page without an account. A free Mailsac account raises the limit and gives you an API key to check signups from your own code.
- Do you keep the emails I check?
- Addresses checked on this page are not saved. We record only that a check happened and its result, to measure how the tool is used.
- Why does Mailsac build this?
- Mailsac has run programmatic inboxes for test automation and AI agents for over ten years, so we see what automated signups look like from the receiving side.
