Sandboxing prevents unwanted emails from entering inboxes by opening attached files in a “safe” environment and testing them for malware. If malware is detected, the email is blocked. GreyListing is a way of checking whether or not suspicious emails are legitimate or spam. The email will temporarily be rejected, and a request will be sent to the original server that sent the email. As spam email accounts are constantly sending emails, it will be too busy to respond to the request and if a certain amount of time has elapsed, the email will be blocked. This allows the spam filter to block emails from senders that might not yet appear on the blacklist.