memcached - Add support for using dev-libs/libmemcached
opendbx - Use opendbx backend to facilitate dataset driven OpenDKIM configuration options like stats, bodylengthdb, etc. against a wide variety of database types
poll - Use poll() instead of select() for the provided asynchronous resolver library.
querycache - Add support for query caching (requires 'berkdb' flag)
sasl - Used to authenticate to a LDAP server in various ways if required.
selinux - !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
ssl - Add support for SSL/TLS connections (Secure Socket Layer / Transport Layer Security)
static-libs - Build static versions of dynamic libraries as well
stats - Add support for statistics (requires 'opendbx' flag)
test - Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
unbound - Use the unbound dnssec library to perform DKIM DNS queries.