Since each request is from a client to a server, the resource record requested works its way back in steps. Once these records get back to the name server, they will be entered into a cache there in case they are needed later. However, this information is not authoritative since changes made will not be propagated to all the caches in the world that may know about it. The query method described here is known as a recursive query since each server that does not have the requested information goes and finds it somewhere else and reports back. If the DNS fails to give a response before its timer goes off, it normally will try another server next time.
Most systems allow users to create mailboxes to store incoming e-mail.
Registered e-mail is used to let the originator know that his message has arrived. Carbon-copies , high priority mail, encrypted mail, alternative recipients, etc. Message Formats RFC 822 Messages consist of a primitive envelope, some number of header fields, a blank line, and then the message body. In normal usage, the user agent builds a message and passes it to the message transfer agent, which then uses some of the header fields to construct the actual envelope, a mixing of message and envelope. In addition, RFC 822 messages may also contain a variety of header fields used by the user agents or human recipients.