Administrator's Guide - Kerio Software Archive

Glossary of termsApplication protocolApplication protocols are conveyed by packets of the TCP or the UDP protocol. It is usedto transfer user (application) data. There are many standard application protocols (e.g.SMTP, POP3, HTTP, FTP, etc.), however, it is possible to develop a custom (non-standard)communication method.DoS attackDoS (Denial of Service) is a type of attack when too many concurrent requests overloadthe system; the server is no more able to respond to the requests of authorized users orfails.DSNDSN (Delivery Status Notification) is an information about the email message delivery status.There are a couple of different types of delivery status notification. Unless otherwisespecified, users receive only the error messages from the mailserver (deferred, failure).Email AddressAn 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 (afterthe @ character). A domain specifies where email be delivered to (a company), a local partspecifies a particular recipient within this domain.ETRNIf you receive email using the SMTP protocol and your server is not permanently connectedto the Internet, email can be accumulated at another SMTP server (typically a secondaryserver for a given domain). When it is connected to the Internet, the SMTP serversends an ETRN command (command of SMTP protocol) and asks for stored emails to betransmitted.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 receiveany emails, it terminates the connection after the specified timeout.FirewallSoftware or hardware device that protects a computer or computer network against attacksfrom external sources (typically from the Internet).IMAPInternet Message Access Protocol (IMAP) enables clients to manage messages stored ona mail server without downloading them to a local computer. This architecture allowsthe user to access his/her mail from multiple locations (messages downloaded to a localcomputer would not be available from other locations).395