Собственно в логах почтовика это
Locale support for language C (C.UTF8) is not installed on the system: locale::facet::_S_create_c_locale name not valid
user@server:~# locale
LANG=ru_RU.UTF-8
LC_CTYPE="ru_RU.UTF-8"
LC_NUMERIC="ru_RU.UTF-8"
LC_TIME="ru_RU.UTF-8"
LC_COLLATE="ru_RU.UTF-8"
LC_MONETARY="ru_RU.UTF-8"
LC_MESSAGES="ru_RU.UTF-8"
LC_PAPER="ru_RU.UTF-8"
LC_NAME="ru_RU.UTF-8"
LC_ADDRESS="ru_RU.UTF-8"
LC_TELEPHONE="ru_RU.UTF-8"
LC_MEASUREMENT="ru_RU.UTF-8"
LC_IDENTIFICATION="ru_RU.UTF-8"
LC_ALL=ru_RU.UTF-8
user@server:~# locale -a
C
POSIX
ru_RU
ru_RU.cp1251
ru_RU.koi8r
ru_RU.utf8