Setup email relay server using sendmail

To setup email relay server using sendmail use following steps:

Install sendmail on relay server. Ensure that incoming and outgoing connections to TCP/25 are allowed. Further DNS queries for MX / A records should work correctly on this machine.
```
yum -y install sendmail
```
Enable on startup
```
service sendmail start
chkconfig sendmail on
```
On CentOS 7 'systemctl start sendmail; systemctl enable sendmail'
Allow access to specific clients by editing '/etc/mail/access'. After adding new IPs using CONNECT for RELAY, do
```
cd /etc/mail
make
service sendmail reload
```
for new settings to take effect.

On CentOS7 use 'systemctl reload sendmail' instead
On clients mention SMART_HOST as FQDN of relay server. In case of postfix clients set relayhost to FQDN of this server

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