How Mail essentials gateway 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. Effectively Mail essentials will act as a mail relay server. Mail essentials works as a mail gateway/relay