Glossary of terms
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## Glossary of terms

DoS attack

  DoS (Denial of Service) is a type of attack when too many concurrent requests overload the system; the server is no more able to respond to the requests of authorized users or fails.

DSN

  DSN (Delivery Status Notification) is an information about the email message delivery status. There are a couple of different types of delivery status notification. Unless otherwise specified, users receive only the error messages from the mailserver (deferred, failure).

Email Address

  An email address determines the sender and recipient of a message in electronic communication. It consists of a local part (before the `@` character) and a domain part (after the `@` character). A domain specifies where email be delivered to (a company), a local part specifies a particular recipient within this domain.

ETRN

  If you receive email using the SMTP protocol and your server is not permanently connected to the Internet, email can be accumulated at another SMTP server (typically a secondary server for a given domain). When it is connected to the Internet, the SMTP server sends an ETRN command (command of SMTP protocol) and asks for stored emails to be transmitted.

  If the given SMTP server doesn't have any messages stored, it doesn't need to reply at all. That's why it is necessary to define a timeout period. If the SMTP server doesn't receive any emails, it terminates the connection after the specified timeout.

Firewall

  Software or hardware device that protects a computer or computer network against attacks from external sources (typically from the Internet).

Free/Busy

  The Kerio MailServer's built-in Free/Busy server is a server using HTTP to provide information on busyness and free time of other Kerio MailServer users without details of individual events being displayed.

IMAP

  Internet Message Access Protocol (IMAP) enables clients to manage messages stored on a mail server without downloading them to a local computer. This architecture allows the user to access his/her mail from multiple locations (messages downloaded to a local computer would not be available from other locations).

  It is possible under certain conditions to access the email account using both IMAP and POP3 protocols.

IP

  IP (Internet Protocol) is a protocol which uses its data part to convey all the other protocols. The most important information in its header is the source and destination IP address, i.e. by which host the packet was sent and to which host it should be delivered.

IP address

  IP address is a unique 32-bit number used to identify the host in the Internet. It is represented by four bytes in the decimal system (`0-255`) separated by dots (e.g. `200.152.21.5`). Each packet includes the information on where the packet was sent from (source IP address) and to which host it should be delivered (destination IP address).

Kerberos

  Protocol for secure user authentication in network environments. It was designed by MIT (Massachusetts Institute of Technology) within the Athena project. The protocol is based on such principles where the third side is trustworthy. Users use their passwords to authenticate to the central server (KDC, Key Distribution Center) and the server sends them encrypted tickets which can be used to authenticate to various services in the network.

LDAP

  LDAP (Lightweight Directory Access Protocol) is an Internet protocol used to access directory services. Information about user accounts and user rights, about hosts included in the network, etc. are stored in the directories. Typically LDAP is used by email applications to search for email addresses and to delivery management (Microsoft Active Directory).

Mailbox Account

  A place where email is stored on a server. Clients can download emails from an account (using POP3 protocol) or work with messages directly at the server (using IMAP or WebMail).

  The account is physically represented by a directory on a disk. The directory is created in the Kerio MailServer directory (`mail/user_name`). Other subdirectories representing individual folders are created in this directory.

  Mailboxes are not created during the definitions of users, the concrete mailbox is created after the first email to this mailbox is received.

MAPI

  MAPI (Messaging Application Programming Interface) is an application programming interface (API) designed by Microsoft. Any software that supports MAPI can communicate with any mailserver (Kerio MailServer) and send and receive data via this interface regardless of their type and software provider.

MX Records

  One of the record types that might be saved in DNS. It includes the information about the mailserver for a particular domain (information about which SMTP server email for this domain should be sent to). Multiple MX records may be defined with different MX preference values to denote priority.

NNTP

  NNTP (Network News Transfer Protocol) is a simple text protocol that allows for distribution, retrieval and posting of messages on the Internet.

Notifications

  Short message (notification) about a particular event — e.g. new email. It is usually sent as a text message (SMS) to a cellular phone.

POP3

  Post Office Protocol is a protocol that enables users to download messages from a server to their local computer. It is suitable for clients who don't have a permanent connection to the Internet.

  Unlike Internet Message Access Protocol (IMAP), POP3 does not allow users to manipulate messages at the server. Mail is simply downloaded to the client where messages are managed locally. POP3 enables access only to the INBOX folder and it does not support public and shared folders.

Port

  16-bit number (`1-65535`) used by TCP and UDP for application (services) identification on a given computer. More than one application can be run at a host simultaneously (e.g. web server, mail client, web client — web browser, FTP client, etc.). Each application is identified by a port number. Ports `1-1023` are reserved and used by well known services (e.g. `80` = WWW). Ports above `1023` can be freely used by any application.

RFC

  RFC (Request For Comments) is a set of deliberately recognized standards. It is a set of indexed documents where each document focuses a particular area of network communication.

SMTP

  Simple Mail Transfer Protocol is used for sending email between mail servers. The SMTP envelope identifies the sender/recipient of an email.

Spam

  Unwanted, usually advertisement email. Spam are usually sent in bulk and the recipient addresses are obtained by illegal means.

SSL

  A protocol used to secure and encrypt the TCP connection. Secure Socket Layer was originally designed by Netscape to secure transmission of web pages using HTTP protocol. Today it is supported by almost all standard internet protocols — SMTP, POP3, IMAP, LDAP, etc.

  At the beginning of communication, an encryption key is requested and transferred using asymmetrical encryption. This key is then used to encrypt (symmetrically) the data.

Subnet mask

  Subnet mask divides an IP address in two parts: network mask and an address of a host in the network. The mask has the same format as IP addresses (e.g. 255.255.255.0), but it is displayed as a 32-bit number with certain number of left-to-right oriented ones and zeros (mask cannot include other values). Number one in a subnet mask represents a bit of the network address and zero stands for a host's address bit. All hosts within a particular subnet must have identical subnet mask and network part of IP address.

TLS

  Transport Layer Security. A later version of SSL, in fact it may be considered as SSL version 3.1. This version is standardized by IETF.

WebDAV

  Using WebDAV (Web Distributed Authoring and Versioning), users can group-edit and organize files located on servers.

WebMail

  Interface used by Kerio MailServer to enable access to email through a web browser. Several user settings (such as message filtering, password, etc.) can be also changed using Kerio WebMail.

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Appendix B Used open-source libraries | Contents | Index