Emailing to more than 250 candidates

Emailing to more than 250 candidates

When I send a mass email to Candidates, it says it will send to the first 250 of my candidates. Two questions: 1. Which is the "first 250?" Alphabetical by first name? Numerical by Candidate ID number? Something else? 2. Will the email go out the following day to the next 250? Thanks, Adam