The MASS working group will produce specifications that permit
authentication of message headers during transit, using public-key signatures
and based on domain name identifiers.
| The MASS working group will produce specifications that permit
| authentication of message headers [together with the validation of
| message content sufficient to curtail simple header replay]. The
| methods developed will use public-key signatures [in a manner
| transparent to the recipients who use existing mail-user-agents]. This
| process is performed during transit [to accommodate existing MUAs,]
| and based on domain name identifiers [to facilitate deployment].
This seems to leave out checking content as a means to address simple
header replay. This attempts to provide some rationale to support most
of your statement.