Remove Mailbox.org

[MMA-block]

@dngray

It did however pass on all other tests with A.

• Deliverability test - Validated: Grade: A
• Fake subdomain protection - Enforced: Grade: A
• Look-a-like protection - Enforced: Grade: A
• Subdomain attack protection - Enforced: Grade: A

I wonder how other providers we recommend compare.

I literally responded with my testing to this comment in this thread…