Administrator's Guide - Kerio Software Archive

Chapter 15Sending and Receiving Mail15.1 Mail Delivery over the InternetUnderstanding the basic principles of mail delivery over the Internet will help you correctlyset your mailserver. This chapter gives a brief overview of the most important information onthis topic. Experienced network administrators can skip this chapter. JMX RecordsAppropriate records must be entered into the DNS (DNS is a world-wide distributed databaseof domain names) for each Internet domain. One of these records is called a MX record (MaileXchanger or the mailserver). An MX record for the domain company.com might look like this:company.com MX 10 mail.company.comMX 20 smtp.provider.commail.company.com A 215.75.128.33smtp.provider.com A 215.75.128.1These records indicate that the mailserver with a preference of 10 is a computernamed mail.company.com and the server with a preference of 20 is a computer namedsmtp.isp.com. Preference means value of the server. The lower the preference the higherthe priority of that server — this implies that the server mail.company.com is the highestpriority mail server for the domain company.com and the server smtp.isp.com is the secondhighest priority mail server for the domain. Arbitrary number of MX records can be definedfor the given domain. If two or more records have the same priority, then one of these serversis chosen randomly (load balancing).The other two records are A type (Address). These tell us which IP address is assigned toa given computer (a MX record can only be assigned to a DNS name, but not an IP address).Email DeliveryHow does an email travel from the sender to the addressee?The sender’s mail client sends the email to its SMTP server. The server checks the recipient’saddress and if the domain contained within the address is qualified as local the email is saveddirectly into the appropriate mailbox. If the domain is not local, the SMTP server finds thename of the primary mailserver (SMTP) for the target domain from the DNS (by sending a DNSrequest) and sends the email to this server. This saves it to a mailbox from which the recipientdownloads it using his/her email client.136