Bluebottle. 100% effective spam protection. It says here. Free and premium versions. Worth a look.
Bluebottle’s Email Verification (Challenge/Response) feature is similar to having your own gatekeeper. It works by not allowing any email to be delivered to your inbox unless the sender.s address is on your allowed list. When an email is received from an address not on your allowed list the email is held and a verification request is returned to the sender. Once the sender responds to the verification request, essentially proving that they are a person and not a spam sending computer, the email that is being held will be delivered to your inbox and the sender will be added to your allowed list