The mail-client/thunderbird-17.0.6 and mail-client/thunderbird-17.0.6-r1 packages both pull in Enigmail in version 1.5.1. However, when creating signed emails with that Enigmail version and using umlauts (e.g. äöü), the character encoding is messed up and it ultimately results in a wrongly encoded mail and consequently broken signature. I've first blamed this on Enigmail and written to their mailing list: "https://admin.hostpoint.ch/pipermail/enigmail-users_enigmail.net/2013-July/000832.html" (subject is "Umlauts botched in signed mails"). However, they advised me to deinstall the 1.5.1 Enigmail that currently is in the ebuild and use the currently available 1.5.2 version (which has this bug fixed). Current workaround: Emerge thunderbird with USE=-crypt, then install Enigmail manually from website (version >= 1.5.2). Please include Enigmail 1.5.2 in the current ebuild.
tb-17.0.7-r1 is in the tree with updated enigmail, will push it for stable in a week.
Setting email composition encoding to utf-8 helped in my case. mailnews.send_default_charset;UTF-8 http://sourceforge.net/p/enigmail/bugs/106/
