Regarding to the exim's spec.txt-file you just have to add HAVE_IPV6=YES to your Local/Makefile. The approriate changes to the ebuild are in the attached patch.
Created attachment 14648 [details, diff] exim-ipv6.patch
exim-4.21 has been added to portage with the ipv6 hooks. Please upgrade and test this new version.
Seems to work now: $ exim -bV Exim version 4.21 #1 built 18-Aug-2003 15:24:38 Copyright (c) University of Cambridge 2003 Berkeley DB: Sleepycat Software: Berkeley DB 4.0.14: (November 18, 2001) Support for: iconv() IPv6 PAM Perl TCPwrappers OpenSSL Thanks! :-)