Atenção:

Para o servidor de e-mail funcionar corretamente é necessário endereço IP público com reverso apontando para o endereço seu servidor. O(s) domínio(s) deve(m) estar registrado(s) nos órgãos competentes.

Após uma instalação simples do Centos 7 (mínima):

Vamos instalar as Ferramentas de desenvolvimento, pois será necessário compilarmos algumas aplicações posteriormente.

yum -y install "Ferramentas de Desenvolvimento"

yum -y install wget whois nc vim gpm dialog logwatch telnet nmap mutt
yum -y install epel-release
yum -y update
yum -y install perl-MailTools perl-MIME-EncWords perl-Email-Valid perl-Test-Pod dovecot dovecot-mysql dovecot-pigeonhole  perl-Mail-Sender perl-Log-Log4perl imapsync offlineimap amavisd-new clamav perl-Razor-Agent mariadb-server opendkim vim crypto-utils mod_ssl php php-mysql php-fpm clamav-update php-imap mailx lrzip lzop lz4 arj  unzoo cabextract p7zip unzip unrar fail2ban php-mcrypt

Presumo que este servidor também  será usado para responder pelo serviço de DNS.
yum -y install bind bind-devel bind-utils
systemctl stop rpcbind
systemctl disable rpcbind

Configure o seu hostname server.
hostnamectl set-hostname mail.seudominio.com.br
echo "192.168.30.2 mail.seudominio.com.br" >> /etc/hosts

Até aqui o servidor já poderá enviar e-mails dos usuários registrados no servidor através de um único domínio.
Lembre-se que o serviço de dns já deve estar configurado, com reverso apontando para essa máquina. Mas vamos continuar, pois queremos que esse servidor de e-mail possa ser usado para mais de um domínio.

PS: No Centos 7 o MySQL-Server foi substituído pelo Mariadb.

Habilite inicie o serviço Mariadb
systemctl enable mariadb.service
systemctl start mariadb.service
mysql_secure_installation

Configurando Clam Antivirus:
vim /etc/sysconfig/freshclam
Comente ou remova esta linha
#  FRESHCLAM_DELAY=disabled-warn    # REMOVE ME

Editando clamav config
vim /etc/freshclam.conf
Comente a linha:
# Example

Faça um update do database.
freshclam

Habilitar o serviço do spamassasin
systemctl start spamassassin.service
systemctl status spamassassin.service
systemctl enable spamassassin.service

Update spamassasin
sa-update

Integrando spamassasin e clamav com o amavisd.
yum -y install clamav clamav-devel clamav-server clamd
cp /usr/share/doc/clamav-server-0.99.2/clamd.sysconfig /etc/sysconfig/clamd.amavisd

vim /etc/sysconfig/clamd.amavisd

vi /etc/tmpfiles.d/clamd.amavisd.conf

systemctl start clamd@amavisd
systemctl enable clamd@amavisd
systemctl status clamd@amavisd

$max_servers = <number>

linha 20 $mydomain
$mydomain = ‘ceae.info’;

linha 152 hostname
$myhostname= ‘mail.ceae.info’;

# ServerAdmin root@localhost

## AllowOverride None
AllowOverride All

;cgi.fix_pathinfo=1
cgi.fix_pathinfo=0

;date.timezone =
date.timezone = America/Sao_Paulo

wget http://downloads.sourceforge.net/project/postfixadmin/postfixadmin/postfixadmin-3.0/postfixadmin-3.0.tar.gz?r=https%3A%2F%2Fsourceforge.net%2Fprojects%2Fpostfixadmin%2Ffiles%2Fpostfixadmin%2Fpostfixadmin-3.0%2F&ts=1479731076&use_mirror=netix


postfixadmin-3.0.tar.gz

tar -zxvf postfixadmin-3.0.tar.gz -C /var/www/html/

cd /var/www/html/

chown -R root.apache postfixadmin-3.0/

 ln -s postfixadmin-3.0/ postfixadmin

mysql -u root -p
MariaDB [(none)]> create database postfix;
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> GRANT all on postfix.* to 'postfix'@'localhost' identified by 'yourPASSword';
Query OK, 0 rows affected (0.00 sec)

$CONF[‘configured’] = true;
$CONF[‘setup_password’] = ‘YOUR-STRONG-PASSWORD’;
$CONF[‘database_type’] = ‘mysqli’;
$CONF[‘database_host’] = ‘localhost’;
$CONF[‘database_user’] = ‘postfix’;
$CONF[‘database_password’] = ‘yourPASSword’;
$CONF[‘database_name’] = ‘postfix’;

$CONF[‘show_password’] = ‘YES’;

$CONF[‘page_size’] = ’30’;

$CONF[‘default_aliases’] = array (
‘abuse’ => ‘[email protected]’,
‘hostmaster’ => ‘[email protected]’,
‘postmaster’ => ‘[email protected]’,
‘webmaster’ => ‘[email protected]’
);

$CONF[‘domain_path’] = ‘NO’;
$CONF[‘domain_in_mailbox’] = ‘YES’;
$CONF[‘maildir_name_hook’] = ‘NO’;

$CONF[‘transport’] = ‘YES’;

$CONF[‘vacation’] = ‘YES’;

$CONF[‘vacation_domain’] = ‘autoreply.ceae.info’;

$CONF[‘vacation_control’] =’YES’;

# The mysqld.sock socket may be in different locations in different systems
driver = mysql
##
connect = host=localhost dbname=postfix user=postfix password=yourpassword
#
# Default password scheme.
# depends on your $CONF['encrypt'] setting:
# md5crypt  -> MD5-CRYPT
# md5       -> PLAIN-MD5
# cleartext -> PLAIN
default_pass_scheme = MD5-CRYPT
# Query to retrieve password. user can be used to retrieve username in other
# # formats also.
password_query = SELECT username AS user,password FROM mailbox WHERE username = '%u' AND active='1'
# Query to retrieve user information.
## user_query = SELECT maildir, 1001 AS uid, 1001 AS gid FROM mailbox WHERE username = '%u' AND active='1'
user_query = SELECT CONCAT('/var/spool/vmail/', domain,'/', maildir) AS
home, CONCAT('maildir:/var/spool/vmail/',domain,'/', maildir) AS mail,
5000 AS uid, 12 AS gid, concat('dict:storage=',CAST(ROUND(quota / 1024)
AS CHAR), '::proxy::quota') AS quota, CONCAT('*:storage=',CAST(quota AS
CHAR), 'B') AS quota_rule FROM mailbox WHERE username = '%u' AND active =
'1'

protocols = imap pop3

#login_greeting = Dovecot ready.
login_greeting = Server OK.

shutdown_clients = yes

auth_verbose_passwords = plain

#auth_debug_passwords = no
auth_debug_passwords = yes

systemctl restart dovecot.service

useradd -r -u 5000 -g mail -d /var/spool/vmail -s /sbin/nologin -c "Virtual mailbox" vmail

mkdir /var/spool/vmail

/var/log/dovecot.log {
missingok
notifempty
delaycompress
sharedscripts
postrotate
/bin/kill -USR1 `cat /var/run/dovecot/master.pid 2>/dev/null` 2> /dev/null || true
endscript
}

disable_plaintext_auth = yes

auth_mechanisms = plain login cram-md5

#!include auth-system.conf.ext
!include auth-sql.conf.ext

ssl_protocols = !SSLv2 !SSLv3

ssl_cipher_list = EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH+aRSA+RC4:EECDH:EDH+aRSA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4

ssl_prefer_server_ciphers = yes

protocol lda {
# Space separated list of plugins to load (default is global mail_plugins).
#mail_plugins = $mail_plugins
mail_plugins = $mail_plugins sieve
}