In contests with voting enabled, by default, all email addresses can be used to vote but we've now added the option to restrict voting to a specific email address or group of email addresses, or to an email domain.

This option is especially thought for voting contests organized for a closed group of users (for example company employees). With this option configured, only users with the allowed email or email domain can vote. 

Note: This option will only show if you select the option of authenticating through email. 

Read this article to learn more details about how the voting system works.