It suffers the same problem as digital signing. It requires that all of your email correspondants use it in order for it to be effective.
As an email user, I don't have control over how my correspondants use email. The only thing I have control over is what email address I give them. By piggy-backing a username and password in the address I give each of my correspondants, I can identify and authenticate them - without buy in from them.
That is the essence of this Segmail spec - and what makes it different from DSPAM, SPF, Statistical Junk Mail Filters, Challenge/Response systems and Digital Signing.