4) Proposals must have IP licensing terms that allow for use in both
commercial and GPLed MTAs.
Again, the PRA RFC has IPR claims. It is my understanding (IANAL)
that there is a very good chance that these license offered with the
PRA is incompatible with the GPL, and may have other problems. There
are significant MTAs out there (e.g. Postfix and Exim) along with spam
filters and MUAs that the current PRA license would prevent the
implementation of SenderID.
If, indeed, the PRA license is incompatible, I do not think we should
advance it as an RFC.
I can't find any statement in draft-ietf-marid-submitter-01.txt that
would support this claim.
Note that to go forward the only requirement is code, and that is strictly
speaking only required to go to draft. There has never been a requirement
for open source code, let alone open source code that is compatible with
any given license.
It should be possible to write code to the spec and release it as public
domain (which is in my view the only real open source license). If you
can do that you can meet any other license term.