Block email subaddresses

Many email providers deliver mail sent to a subaddress, such as user+1@example.com, to the same inbox as user@example.com. Someone can use this to create many accounts from a single inbox. This protection blocks an email address when another address that differs only by its subaddress already has an account.

What it blocks

When someone signs up with an email address, or adds one to an existing account, Clerk removes the subaddress and checks whether the resulting base address is already registered. If it is, the new address is blocked. For example, if user@example.com already has an account, user+1@example.com is blocked.

Email providers use different formats for subaddresses, so Clerk applies different rules depending on the provider:

The first sign-up for a base address is allowed, even if it uses a subaddress. Only later sign-ups that use another variation of the same base address are blocked.

Existing accounts aren't affected. Users who already have an account with a subaddress can keep signing in.

What happens when an address is blocked

The user sees an error that the email address is already in use, and can't continue with that address.

Settings

This protection has no settings.

How it works with other protections

Block disposable email addresses blocks throwaway inboxes. This protection blocks extra accounts from one real inbox. They cover different tricks, so you can turn on both.