Here are two lists of secure (at date of writing) ciphers for the CentOS/RHEL operating system. These lists are not complete but only reduced to RSA/AES encryption ciphers which are considered secure. Use following command to get all supported ciphers on your OS:

openssl ciphers -v ALL

Following string uses secure ciphers while keeping some of the older secure ciphers available. This string can be used in apache, postfix or others:

ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5

CentOS 6+7:

ECDHE-RSA-AES256-GCM-SHA384
ECDHE-RSA-AES256-SHA384
ECDHE-RSA-AES256-SHA
DHE-RSA-AES256-GCM-SHA384
DHE-RSA-AES256-SHA256
DHE-RSA-AES256-SHA
ECDH-RSA-AES256-GCM-SHA384
ECDH-RSA-AES256-SHA384
ECDH-RSA-AES256-SHA
ECDHE-RSA-AES128-GCM-SHA256
ECDHE-RSA-AES128-SHA256
ECDHE-RSA-AES128-SHA
DHE-RSA-AES128-GCM-SHA256
DHE-RSA-AES128-SHA256
DHE-RSA-AES128-SHA
ECDH-RSA-AES128-GCM-SHA256
ECDH-RSA-AES128-SHA256
ECDH-RSA-AES128-SHA

CentOS 5:

DHE-RSA-AES256-SHA
DHE-RSA-AES128-SHA