Email validation APIs from providers like Mailgun, Kickbox, AbstractAPI, Email Hippo, Debounce.io, and NeverBounce can be integrated into signup forms to identify and reject invalid email addresses in real-time. These APIs verify email syntax, domain status, mailbox existence, and check for disposable emails, typos, and other indicators of fake accounts. This process helps filter out risky or invalid signups, enhances signup security, and improves overall data accuracy.