Email Forwarder Checker
Find out if a domain is an email forwarding service like SimpleLogin, AnonAddy, or DuckDuckGo Email, where one person can create many working addresses.
REST API
All of UserCheck's email and domain intelligence in a fast and simple API.
$ curl https://api.usercheck.com/email/[email protected]
{
"relay_domain": true
}1,000 free credits each month · no credit card required.
How email forwarders work
An email forwarder gives a user an alias, like [email protected] or [email protected], and relays anything sent there to their real inbox. The user never exposes their actual address and can turn the alias off at any time. For a signup flow, the relevant property is that one person can create any number of distinct, working addresses.
This matters for trial limits: if you cap free trials at one per email address, every new alias looks like a new user but lands in the same mailbox. The same property shows up in chargeback fraud and multi-accounting, where a forwarder keeps the account reachable without identifying its owner.
A forwarder isn't a disposable address, though, even when it looks like one. The inbox behind it is real, the person is reachable, and the alias often sticks around for years. UserCheck reports it as its own signal, separate from disposable, so you decide how much weight it carries.
Who runs these services
The big ones are SimpleLogin, addy.io (formerly AnonAddy), DuckDuckGo Email Protection, Apple's Hide My Email, Firefox Relay, and 33Mail. Several privacy-focused mailbox providers, including Proton, hand out aliases too. UserCheck tracks them as they appear.
Checking it from your code
Both the /domain and /email endpoints return relay_domain: true or false. Add a step-up check or a flag when it's true, in one call. See the API documentation for examples.
Common questions
What is an email forwarder?
A service that gives a user an alias, like [email protected], and relays anything sent to it to their real inbox. The user can disable the alias whenever they want. You'll also see it called email relay, masked email, or aliasing.
Are forwarded emails disposable?
No. A disposable address is a public throwaway anyone can read, like Mailinator or 10MinuteMail. A forwarded address is a private alias controlled by one real person, with the mail landing in their actual inbox. UserCheck reports the two separately so you don't have to treat them the same.
Should I block email forwarders at signup?
Depends on the product. Plenty of real customers use forwarders for privacy, so a blanket block costs you legitimate signups. On trial-sensitive products and marketplaces, step-up verification on forwarded signups is the usual middle ground. Elsewhere, flag and move on.
Which forwarding services does UserCheck detect?
The major ones: SimpleLogin, addy.io, DuckDuckGo Email Protection, Apple Hide My Email, Firefox Relay, and 33Mail, plus the aliases handed out by privacy providers like Proton. New services get added as they show up in API traffic.
Can I tell which forwarding service an address uses?
Often. When a domain routes its mail through a recognized relay, that shows up in mx_providers with a forwarding type, like ImprovMX or Cloudflare Email Routing. relay_domain tells you it's a forwarder; mx_providers can tell you which one.
Can I check forwarder status from my code?
Yes. Both /domain/{domain} and /email/{email} return relay_domain: true|false. Trigger a step-up check or a flag when it's true. The API documentation has examples.