It's nice to play in the sandbox but at some point you might want wider testing.
Guile 1.9.* is still in beta state. We've been hearing "This may be the last release before 2.0 comes out" since early this year, but things haven't changed. So we were just waiting for an official stable release. Version 1.9.11 is in the overlay for some time now, but I'm not in contact with guile users to see if it works properly. Have you tested this?
I think we can think about adding >=1.9.13 to main tree.
This is not going to happen. I will add a 2.0 version some time in the near future though (bug #355355).
Marking this as duplicate of (instead of wont-fixing) request for guile-2, since net-mail/mailutils-2.2 has optional support for it. (Which would have been more obvious if it had been stated previously instead of the comments about testing). *** This bug has been marked as a duplicate of bug 355355 ***