What are ptr, exists, exp, and macro mechanisms?

• These are advanced and rarely needed.
• ptr checks reverse DNS. It is discouraged because it is slow and unreliable.
• exists evaluates whether a domain resolves. Useful for dynamic validation.
• exp points to an explanation string for failures.

Macros are dynamic tokens that expand during evaluation. Mostly used by large ESPs and security systems.

These mechanisms exist but are avoided in most practical deployments because they complicate SPF.