Para instalar mcrypt en Centos, hay que seguir los siguientes pasos. 

wget http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

wget http://rpms.famillecollet.com/enterprise/remi-release-6.rpm

sudo rpm -Uvh remi-release-6*.rpm epel-release-6*.rpm

sudo yum update

sudo yum install php-mcrypt*

Editar el fichero mcrypt.ini y agregar la siguiente línea

sudo nano /etc/php.d/mcrypt.ini

extension=/usr/lib64/php/modules/mcrypt.so

sudo service httpd restart

En Centos, hay que seguir los siguientes pasos para instalar SPAMASSASSIN

## screen -U -S spamc-screen

## yum update

## yum install spamassassin

## groupadd spamfilter

## useradd -g spamfilter -s /bin/false -d /usr/local/spamassassin spamfilter

## chown spamfilter: /usr/local/spamassassin

Ahora toca configurar SPAMASSASSIN

## vim /etc/mail/spamassassin/local.cf

## Required_hits: This determines the filter balance; the lower the score the more aggressive the filter.
# A setting of 5.0 is generally effective for a small organisation or a single user.
# Adjust the strictness score to your organization's needs - a large medical organisation might want  to let email items
# through that are trying to sell pharmaceuticals, so we might increase the level to a more modest 8.0.
required_hits 5
## Report_safe:  This line determines whether to delete the item or to move the item to the inbox whilst appending
# a spam notice to the subject line. The levels for this line are set to either a 1 or 0. A score of 1 will delete the spam item,
# whereas a score of 0 will send the item to the inbox and rewrite the subject line.
report_safe 0
rewrite_header Subject [**SPAM**]
## Required_score: This line sets the spam score for all email allowed through to your domain, with levels of certainty set from 0 to 5.
# Zero would be classified as a legitimate email item, whereas 5 would be an definite 'SPAM' item.  If we set the score to 3 we would catch a
# lot of unsolicited emails but quite a few false positives would still get through. For our example email server we will use the score of 5,
# but you can of course set this value according to your preference.
required_score 5.0

## vim /etc/sysconfig/spamassassin

Agregar lo siguiente

SAHOME="/usr/local/spamassassin"
SPID_DIR="/var/run/spamassassin"
SUSER="spamfilter"
SPAMDOPTIONS="-d -c -m5 --username ${SUSER} -H ${SAHOME} -s ${SAHOME}/spamfilter.log"

## service spamassassin start

## chkconfig spamassassin on

AHORA HAY QUE CONFIGURAR POSTFIX

## vim /etc/postfix/master.cf

Agregar las siguientes líneas en el fichero.

smtp      inet  n       -       n       -       -       smtpd -o content_filter=spamassassin
spamassassin unix -     n       n       -       -       pipe user=spamfilter argv=/usr/bin/spamc -f -e  /usr/sbin/sendmail -oi -f ${sender} ${recipient}

Y luego reiniciar postfix

## service postfix restart