Most email servers support routing of mail for certain domains to 'smart hosts' rather than using DNS to resolve the mail server. By setting up a route to send all mail for the esendex.net domain to the host ems.esendex.net, you can then send emails to <mobile number>@esendex.net, or <mobile number>@voice.esendex.net, which are then converted into text or voice SMS messages.
The service is able to authenticate the inbound emails and thus authorise the sending of the SMS messages in two ways