In order to combat spam and fraudulent email it's become common practice to deploy technologies such as SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) to provide a means for receivers to verify whether a message is from who it claims to be. Unfortunately, these are not foolproof and can still allow fraudulent messages to slip through. There's also no way of knowing what a receiver will do with messages that fail these checks and no way of knowing how effective they are. DMARC (Domain-based Message Authentication, Reporting and Conformance) goes some way towards improving this.
