How to block all emails from all unknown senders

Unfortunately, completely blocking all emails from unknown senders isn't possible with most email providers. This is because legitimate emails from new contacts you haven't corresponded with before would also be blocked.

However, some advanced techniques can significantly reduce unwanted emails:

1. Spam Filtering: Most email providers have built-in spam filters that are constantly learning and improving. Make sure your spam filtering is set to the highest level. This will catch a large percentage of spam emails before they reach your inbox.
   1. Report and reduce spam in iCloud Mail - Apple Support (IN)
   2. Add custom spam filters to Gmail
   3. Change the level of protection in the Junk Email Filter
   4. Manage spam and mailing lists in Yahoo Mail - SLN28056
   5. Manage spam and privacy in AOL Mail
   6. Configure anti-spam policies in EOP
2. Address Rules: Some email providers allow you to create address rules. While you can't block all unknown addresses, you can create rules to move emails from addresses not in your contacts list to a specific folder, like "Spam" or "For Review". This way, you can easily check these emails and delete them if unwanted. Mail app - Email Rules - Corporate Profil… - Apple Community
3. Domain Blocking: If you're constantly getting spam from a specific domain (e.g., @example.com), you can block the entire domain. This will prevent any emails from that domain from reaching your inbox. Be cautious with this approach, though, as some legitimate senders might use that domain.
4. Sender Authentication: Many email providers support sender authentication protocols like SPF, DKIM, and DMARC. These protocols help verify the legitimacy of the sender's email address. While enabling these features won't directly block emails, it can help your spam filter identify and block spoofed emails pretending to be from someone else.