Who defines and enforces these standards?

Multiple organizations define and enforce email standards at different levels. The IETF (Internet Engineering Task Force) publishes RFCs defining technical protocols. These are authoritative specifications that email software must implement correctly to function.

Industry organizations like M3AAWG (Messaging, Malware, and Mobile Anti-Abuse Working Group) and CSA (Certified Senders Alliance) develop best practice guidelines. These are voluntary but influential standards that major mailbox providers and ESPs expect senders to follow.

Enforcement happens through mailbox providers who implement filtering based on standards compliance. Messages failing authentication or violating behavioral standards face rejection or spam filtering. This market based enforcement makes standards compliance practically mandatory for deliverability.