What tools or configurations can I use to make sure my emails land in the primary inbox instead of the spam folder?

We ask the same underlying question in different ways.

• What tools or configurations can I use to make sure my emails land in the primary inbox instead of the spam folder?
• How can I configure my email settings to avoid the spam folder and ensure my messages reach the primary inbox?
• How can I configure my email sending to ensure it hits the primary inbox and avoids the junk folder?