The marketing team wants to identify customers who are not on the email blocklist and may be considered for campaigns through other contact channels.
The customers table stores customer email addresses, while blocklist contains emails that must be excluded. Blocklist entries may use different capitalization, so email comparisons are case-insensitive.
Some customers have no email recorded. For this exercise, those customers should remain in the report, because the campaign may reach them through another channel. A missing email is not treated as a match to the blocklist.
Ordering: Sort by customer_id in ascending order.