Composition of e-mail box

A letter consists of an envelope and contents. The content of the letter is divided into two parts: the first part and the text part. The letterhead contains attributes about the characteristics of the letter. The text is the information that the sender wants the receiver to obtain, and usually consists of one or more paragraphs. Each channel can have different types of coded information. Envelopes of letters can be divided into three forms: submission envelopes, delivery envelopes and relay envelopes. The sending user interacts with UA to prepare the text of the letter, and UA adds a header to it according to the user's input to indicate the sender, recipient, sending time, letter title and other related attributes of the letter, and then pastes the submission envelope. The submission envelope indicates relevant control information during the submission process, such as which mail transfer agent (MTA) to submit. MTA analyzes and modifies some control information on the delivery envelope to make it a relay envelope, and selects the next MTA according to the address on the relay envelope until it reaches the MTA connected with the receiving UA, thus forming a delivery envelope. Finally, the receiving UA removes the envelope and stores the contents of the letter in the recipient's email box for the recipient to retrieve and copy.