Validate an email address before you accept a signup or send to it: RFC 5322 syntax, whether the domain resolves and its MX records, whether it is a disposable/throwaway domain (community blocklist), and whether it is a role account (info@, support@…). DNS is checked over DoH. No SMTP probing (intrusive and spammy), so a true 'domainExists' does not guarantee the mailbox exists.