Email fields expose SaaS applications to abuse through aliases, relay services, and disposable domains. Attackers exploit these to create multiple accounts, bypass bans, and abuse free tiers.

Gmail, ProtonMail, and other providers allow dot-ignoring and plus-addressing that create infinite aliases. Relay services like Firefox Relay and Apple Hide My Email offer anonymous addresses that forward to real inboxes.

Blocking disposable email providers requires maintaining 38,000+ domain lists. MX record validation helps detect fake domains, while proper regex prevents format bypass. Implementation order matters: cheap checks first, DNS lookups last.