According to the Debian sSMTP page the file permission should be '0640' and belonging to the 'root:mail'.
VERY IMPORTANT !!! If other people have access to this computer
Your GMAIL Password is left unencrypted in this file
so make sure you have a strong root password, and make sure
you change the permissions of this file to be 640:
chown root:mail /etc/ssmtp/ssmtp.conf
chmod 640 /etc/ssmtp/ssmtp.conf
According to the Debian sSMTP page the file permission should be '0640' and belonging to the 'root:mail'.