well, let's try with a custom virtual/mta ebuild that recognizes qmail-jms1 as an mta

virtual/mta/Manifest

@@ -0,0 +1 @@
+EBUILD mta-0.ebuild 1008 RMD160 78f6a87a25ac77bebddf3ce3ec1ab8f445f6a41d SHA1 fe8c31a9d7fd9029dd4e6a19d92ab7a1987ad032 SHA256 c2985e7bcd3c8dbff7c26ee613a3e28970742a32f8fdf8a6524a83483032f5a7

virtual/mta/mta-0.ebuild

@@ -0,0 +1,34 @@
+# Copyright 1999-2011 Gentoo Foundation
+# Distributed under the terms of the GNU General Public License v2
+# $Header: /var/cvsroot/gentoo-x86/virtual/mta/mta-0.ebuild,v 1.5 2011/04/24 06:40:34 eras Exp $
+
+EAPI=3
+
+DESCRIPTION="Virtual for Message Transfer Agents"
+HOMEPAGE=""
+SRC_URI=""
+
+LICENSE=""
+SLOT="0"
+KEYWORDS="alpha amd64 arm hppa ia64 m68k ~mips ppc ppc64 s390 sh sparc x86 ~sparc-fbsd ~x86-fbsd ~x64-freebsd ~x86-freebsd ~x86-interix ~amd64-linux ~x86-linux ~ppc-macos ~x64-macos ~x86-macos ~sparc-solaris ~x64-solaris ~x86-solaris"
+IUSE=""
+
+DEPEND=""
+
+# mail-mta/citadel is from sunrise
+RDEPEND="|| (	>=mail-mta/ssmtp-2.64-r2[mta]
+				<mail-mta/ssmtp-2.64-r2
+				mail-mta/courier
+				mail-mta/esmtp
+				mail-mta/exim
+				mail-mta/mini-qmail
+				>=mail-mta/msmtp-1.4.19-r1[mta]
+				<mail-mta/msmtp-1.4.19-r1
+				mail-mta/nbsmtp
+				mail-mta/netqmail
+				mail-mta/nullmailer
+				mail-mta/postfix
+				mail-mta/qmail-ldap
+				mail-mta/qmail-jms1
+				mail-mta/sendmail
+				mail-mta/citadel[-postfix] )"