How Mail essentials works In order for Mail essentials to check all inbound and outbound mail for the rules you specify, it must be able to analyse all inbound and outbound mails. Therefore, it must be the first to receive all mails destined for your mail server and it must the last 'stop for outbound mail, i.e. mails destined for the internet. In order for this to happen, Mail essentials must act as a gateway for all email. This set-up is also known as 'Smart host' or 'Mail relay' server. Mail essentials works as a mail gateway/relay We recommend that you install Mail essentials on a separate machine in front of your firewall. This allows you to keep your corporate mail server behind the firewall. Mail essentials will act as a smart host/mail relay server in the perimeter network (also known as DMZ, demilitarized zone, and screened subnet). Additional advantages are: You can perform maintenance on your mail server machine, whilst still receiving email from the internet. You use less resources on your mail server machine The Mail essentials machine can have a lower spec then the Exchange server machine and process mail faster Additional fault tolerance – if anything happens with your Exchange server you still receive mail, which is queued on the mail essentials machine until your mail server machine is back up again. If you do not wish to dedicate a machine to Mail essentials, you can install Mail essentials as a mail relay server on your mail server machine itself. To do this you will need to change the port of your mail server from 25 to for example 26.