In spam filtering, whitelists and blacklists are tools used to control which messages or senders are allowed or blocked. Here’s an explanation of each:
Whitelist

A whitelist is a list of approved email addresses, domains, or IPs that are explicitly allowed to send emails to the recipient’s inbox, bypassing spam filters.
• Purpose: To ensure that important emails from trusted sources are never marked as spam.
• Examples:
o Adding your company’s domain to a whitelist to ensure business emails are delivered.
o Allowing a specific sender’s address to ensure their emails bypass spam checks.

Blacklist
A blacklist is a list of email addresses, domains, or IPs that are blocked because they are identified as sources of spam or malicious content.
• Purpose: To prevent emails from unwanted or harmful senders from reaching the recipient’s inbox.
• Examples:
o Blocking known spam domains.
o Adding a persistent spammer’s email address to a blacklist.

How They Work Together
• Spam filters check incoming emails against both whitelists and blacklists:
1. If an email is from a sender on the whitelist, it is automatically allowed.
2. If an email is from a sender on the blacklist, it is blocked or sent to the spam folder.
3. Emails from unknown or unlisted sources are evaluated by other filtering rules (e.g., content analysis, reputation scoring).
Using these lists effectively helps strike a balance between allowing desired emails and blocking spam.