Administrator's Guide - Kerio Software Archive

Chapter 15 Sending and Receiving MailInternet. Under normal circumstances, all messages for this domain are sent to the primaryserver smtp.company.com. If failure of this server occurs (overloading, disconnectedline etc.), all messages are sent to the secondary server smtp2.company.com. When theprimary server becomes available it can send an ETRN command to the secondary server torequest stored mails. Communication is the same as in the previous example (for detaileddescription of secondary SMTP server settings, see chapter 27.5).Mail delivery is faster and more reliable in this way than waiting till the secondary serversends the mails itself (see section Email Delivery). In addition, the ETRN command can beused also for dial lines.domain mailboxThe domain’s primary mailserver does not always need to be the server where user mailboxesare stored. If the company to which the domain is registered connects to the Internet viaa dial-up line, it can have a Domain Mailbox at its ISP. A domain mailbox is an account wheremail for the entire domain is stored. The company’s mailserver can retrieve mail from thismailbox (in certain time intervals) and sort the email into individual user mailboxes. The ISP’sSMTP server, where the domain mailbox is stored, is listed as the primary mailserver for thecompany’s domain in the MX records.Domain mailbox receives the messages via SMTP protocol. Each message therefore containsthe body as well as the SMTP envelope. Only the body of the message is downloaded to thedomain mailbox. The envelope information is copied to a message header (depending on thedomain mailbox settings).Kerio MailServer performs authentication to the domain mailbox. Then it downloads messagesvia POP3 and sorts them according to the rules specified in Kerio MailServer. In order forthe rule to be sorted properly, it must contain the recipient information (either in any of thespecial message headers or in the To or Cc fields). If there is no information about the recipientcontained in the message, the system returns it to the sender. However, if a special sortingrule is created in Kerio MailServer (see chapter 15.4), the messages without any recipient datawill be stored in a predefined user mailbox.Note: It is recommended to specify a special X-Envelope-To: header for message sorting,because it contains information about recipients. This helps you avoid situations where a messageaddressed to multiple users is delivered several times according to the number of recipients.Access of email clients to user accountsUser can use various methods to access their email accounts:POP3POP3 (Post Office Protocol version 3) is an Internet protocol used for downloading ofemail from a server to another server (see the Domain Mailbox section) or to an emailclient. POP3 protocol is defined in RFC 1939.138