При отваряне или препращане на писма с кирилица понякога не се прави коректно от squirrelmail затова е необходимо да конвертираме всичко в utf8 и да го сетнем.

Ето го и решението:

cd /usr/share/squirrelmail_nuts;
find . -iname squirrelmail.po|grep bg
cd locale/bg_BG/LC_MESSAGES/;

iconv -f CP1251 -t UTF-8 squirrelmail.po > squirrelmail.utf-8.po;
sed 's/cp1251/UTF-8/' squirrelmail.utf-8.po > squirrelmail.po;
rm squirrelmail.utf-8.po;
msgfmt -o squirrelmail.mo squirrelmail.po;

cd /usr/share/squirrelmail_nuts;
find . -iname i18n.php

vim ./functions/i18n.php
$languages["bg_BG"]["NAME"] = 'Bulgarian';
$languages["bg_BG"]["CHARSET"] = 'utf-8';
$languages["bg_BG"]["LOCALE"] = 'bg_BG.UTF-8';
$languages["bg"]["ALIAS"] = 'bg_BG';

vim  locale/bg_BG/setup.php
$languages['bg_BG']['NAME']    = 'Bulgarian';
$languages['bg_BG']['CHARSET'] = 'utf-8';
$languages['bg_BG']['LOCALE']  = 'bg_BG.UTF-8';
$languages['bg']['ALIAS']      = 'bg_BG';

/etc/init.d/apache2 reload

thx to Hipo