Experimental Features
From time to time, experimental features may be added to Exim. While a feature is experimental, there will be a build-time option whose name starts "EXPERIMENTAL_" that must be set in order to include the feature. This file contains information about experimental features, all of which are unstable and liable to incompatibile change.
Experimental features are not documented in the mail Exim docs, but the latest documentation on them is available in the distribution: exim/exim-doc/doc-txt/experimental-spec.txt
Yahoo DomainKeys
Sender Policy Framework SPF
Sender Rewriting Scheme SRS