I hate to keep harping on this, but I really do believe that it is important.
I solicited the opinion of Dr. Philip Hazel of Cambridge University who as you
probably know is the principal author of Exim. I asked him if he would
consider releasing Exim under a difference license other than the GPL if doing
so would allow Sender-ID code to be included in it. His response was:
"The short answer is "No".
I have no desire to change Exim's licence, and certainly not as a result
from pressure of Microsoft's lawyers. Furthermore, some of the other
postings I've seen have said that developers using this "free" licence
must register with Microsoft. That is completely unacceptable."
I believe that Dr. Hazel's response demonstrates two things. The first is
that one of the most popular MTAs in use will not be able to include Sender-ID
code. The second is that a very well respected open source developer sees the
IPR license terms as "completely unacceptable".