Administrator's Guide - Kerio Software Archive

15.1 Mail Delivery over the InternetIf the primary mailserver for the target domain is not accessible, the sending SMTP server triesto contact the secondary server (the server with the next priority) and send the email there. Ifno server listed in the MX record for the target domain is accessible the SMTP server will try tosend the mail again repeatedly in defined intervals. If it does not succeed after a certain timethe email is returned to the sender as undeliverable.If, for example, only the secondary server is accessible the email is sent to this secondaryserver. In principle, any SMTP server can function as a secondary (tertiary, etc.) server fora domain.Sending Email via a Different SMTP Server (Relaying)There is also another way email can be delivered to addressees. The client sends the emailmessage to its SMTP server. This server forwards it to another SMTP server which delivers itto the target domain as described above. This method of delivering email is known as relaying(passing to the relay server).The advantage of this relaying is that sending email is an on-off action. Furthermore, emailcan be placed in a queue and sent in defined time intervals. The sending SMTP server doesnot need to ask the DNS about the target domains’ mailservers or try to send the email againif the target servers are inaccessible. This is important mainly for slow or dial-up Internetconnections and it can significantly decrease costs of such connections.Most SMTP servers on the Internet are protected against relaying to prevent misuse of serversfor sending spam email. If you wish to send email via a different SMTP server, you shouldcontact the server’s administrator and ask them that relaying be enabled for you (usuallybased on checking your IP address or using username/password authentication).ETRN CommandETRN is a command of SMTP protocol. It serves for requesting emails stored on another SMTPserver. Typically, it is used in the following situations:1. The client has its own domain (e.g. company.com) and his server is connected to theInternet via a dial-up line. Dial-up must have a fixed IP address. The primary MX recordfor the domain company.com is directed to the ISP’s SMTP server (e.g. smtp.isp.com).When it is connected to the Internet, the client’s SMTP server sends an ETRN commandthat informs that it is online and ready to receive mail. If the primary server has someemails for the given domain, then it sends them. If not, it can send a negative responseor it need not reply at all. That’s why the client’s server must have the timeout to specifyhow long it will wait for the response from the primary server.Note: The primary server will create a new connection to the client’s server after the ETRNcommand reception. This connection is used for mail transmission. If the client’s serveris protected by firewall, TCP port 25 must be accessible (open) to the Internet.2. Let’s suppose that the domain company.com has a primary server smtp.company.com anda secondary server smtp2.company.com. Both servers are permanently connected to the137