Protect from trial abuse
You already know which domains are risky. Block signup from those emails using the Blacklist API.
Email Blacklist for signup and email campaign protection
Block unwanted emails and domains in real time and reject risky registrations using a single API response.
No credit card required. Test it with free credits.
Fast setup
Use API to block unwanted emails
1 field
Use `block` field for allow/deny
Email or Domain
Add exact emails or full domains
API or dashboard
Manage blacklist via API or dashboard
Why founders use Blacklist API
Block unwanted emails directly in API response without writing and maintaining custom blocking logic across backend services.
Clean your mailing list
Remove unwanted emails from your email list and save money on email campaigns using the Blacklist API.
Save money
Block unwanted signups at registration, use less server capacity, fewer third-party API calls, and reduce wasted operational costs.
Blacklist Flow
How Blacklist works
Add risky emails or domains once, then use the `block` field in each verification response to decide whether to allow signup.
01→
Add blacklist rules
Add exact emails like `[email protected]` or full domains like `example.com` in dashboard or API.
02→
Verify signup input
Call `/v1/verify` with user email at registration and receive the full verification data.
03✓
Decide using `block`
If `block: true`, reject signup.
Verification API response example
GET /v1/[email protected]\nX-API-Key: your-api-key\n\n{\n "valid": true,\n "block": true,\n "domain": "rambler.ru"\n}What you can add to the Blacklist
Exact email
Domain
`example.com`
Builders who rely on Blacklist
Real founders and product owners using the `block` field to keep fake signups out while keeping good customers in.
EM
Elena Morozova
Founder, AI startup
“After we blocked a handful of obvious abuse domains, trial abuse dropped and we stopped burning resources on free plans that never convert. It was a small change that finally made trials cost what they should.”
JK
Jonas Klein
CEO, marketing agency
“With Blacklist API we cleaned our email list from useless addresses and now save real money on every campaign we send.”
MR
Marta Rossi
Product Owner, SaaS platform
“Using the `block` field we simply do not let obvious abusers register, so new accounts are real customers instead of throwaway profiles.”
Blacklist API FAQ
Which field tells us an email is on the blacklist?
Use the `block` field from the verification response. This field already includes blacklist and whitelist rules.
Can we blacklist a full domain?
Yes. Add a domain like `example.com` and all emails on this domain can be blocked automatically.
Can we blacklist a single email?
Yes. You can add exact addresses like `[email protected]` for precise blocking rules.
Does `valid` mean we should allow signup?
Not always. `valid` is about email validity and deliverability checks. For allow/deny policy use `block` field.
Can we test Blacklist before paying?
Yes. Start with free trial credits and test blacklist behavior in real requests.