if you own a linux server look at TMDA.
It's a whitelist system which works the following:
1.) New mail arrives on the server
2.) TMDA checks if the sender is on the black list. If so it will be deleted.
3.) If the mail wasn't delted TMDA then checks if the sender is on the white list. If so, then the mail will be put into your inbox.
4.) If the sender's email is on neither list then it will be hold back as pending. TMDA then sends out an automated email which asks the sender to verify his email address. Once done you will receive the email and the sender's email address will be whitelisted!
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