I. Rationale of email
EmailMail systemComposed of the uma->outgoing queue--> news transfer agent message transfer agent --> mailbox. The processes are as follows:
Client machine sends mail to output queue through mail user agent, pending dispatch
2. Mail is transmitted through the sender's transmission agent and to the recipient's transmission agent mta
3. Mta to mailbox - user agent mua to the recipient, ultimately to the client machine
Of which, the mail core
1. Mta is responsible for completing the storage and forwarding of mail
2. Connect between the two mtas through tcp agreements
3. The main protocols used by mta are smtp, pop3
Email coding standards
General e-mails are sent to the recipient through multiple mainframe connections and transmission services. Mail may be destroyed if multiple host-to-host codes differ, such as ascii or 8-bit clean, respectively. So whether the message is in chinese or in pictures, it is coded before it is sent。
Common e-mail codes are:
Uu-code unix-to-unix encoding using uu-coded contents like ? 7t
/*o/
"tl+ku*w
2 mime code multipropose internet mailextension, multimedia mail delivery mode, which can transmit multimedia files in various formats. Two types of code b are defined in mime codeQuote-printable。
1) qp applies to non-ascii-coded text elements, e. G. Chinese files, which are similar to "=c2=d2=db=ad=c0 " after coding, which means that eight characters are expressed as 3-bit (1 = and 2 hexadecimals) and one qp is typically expressed as two eight-digit characters, so the qp is encoded as six characters, e. G. "=c2=d2" as "disordered"。
2) bCase64 is generally used to encode documents sent in an attachment with the word "content-transfer-encoding:b"See also para。
Encoding does not affect the size of the file after the encoding, and most email software with mime encoding functions automatically determines how the mail is coded, and then automatically selects bOkay, let's go。
Email protocol
1. Pop3 agreements
Post office protocol version 3, now commonly used in the third edition, so short pop3. The pop3 protocol allows the client to extract mail from the server. For e-mail reception. Pop3 using the clean/server working model. When the client needs services, the client's software (outlook express or foxmail) will be connected to the pop3 server by tcp, which will then pass through the three working states of the pop3 protocol
1) authentication status: first, authentication process, confirmation of user names and passwords provided by the client machine
2) processing status: upon adoption of the authentication, the user can receive his/her own mail or remove it。
3) update status: after the response operation, the client sends a quit command and thereafter enters the status update, removing the deleted message from the server. The entire pop process has been completed。
Client's common command:
1) stat retrieving mailbox status like number and size of mail
2) retr msg downloads specified mail
3) dele msg will specify mail tags to delete
4) quit end session
Imap agreements
Internet message access protocol, a protocol to access information via the internet. Imap, like pop, provides easy mail-receiving downloads that allow users to read offline, but much more can be done. The summary browsing function provided by imap allows you to make a decision whether or not to download all mail arrival times, themes, senders, sizes, etc. This means that when the imap client on the pc machine supports the first message to see the mail, the mail is sent to the user mailbox when the user needs to open an email. Mail is kept on the imap mailbox server (a mailbox server supporting the imap protocol) until the user has given the order to delete the mail。
3. Smtp agreement
Simple mail transfer protocol, which defines protocols for mail transmission, based on tcp application layer protocols. The objective is to provide efficient and reliable mail transmission to users. It is capable of relaying mail during transmission, i. E., mail can be transmitted via the mainframe mode on different networks. Work is done in two cases: e-mail transmission from the client machine to the server; and two from one server to another. Smtp is a request/response protocol that listens to port 25, receives user mail requests and is connected to a remote mail server。
Client's common command:
1) the helo client issues an order to the other mail server indicating itself。
2) mail from used to identify sender's email address
3) rcpt to indicates the recipient's e-mail address, indicating that if the recipient is not a local user, it wishes the other mail server to forward relay the mail to itself, indicating that the mail server is open relay if the mail server allows for forwarding. Otherwise the mail server does not allow relay。
Normally, login can be used to read mail directly by a client such as mail, but if you want to read mail using local mua e. G. Foxmail, outbook, the local client will interact with the mail server via a pop3 or imap protocol and send the mail to the local client (foxmail)
Iv. Web e-mail system structure and realization
1. Structure
The web mail system is a combination of email and www mail systems, and the client receives and sends mail through the browser。
Client (browser) - >www server (server extension mua) - pop3->mta - > mailbox
1) users access www servers through browsers, while www servers perform mua functionality by extension (e. G. Using asp\php, etc.)
2) mua visits mta to get mail and mta takes mail from the mailbox for transmission
3) mua returns to client browser via www server
2. Achieved
1) dechi fulfils the mta mail server end functionality - each user is managed using a separate directory, saves according to date and provides detailed log management. Users were provided with filtering of mail, group mail, managing mailing records, etc
2) php achieves mua client - with php achieves pop3 protocol, mime code and qp code, enabling users to perform basic functions of email mail system in browser environments。
By functionality, mail servers are divided into smtp servers (mta transmission) and pop3 servers (mua mail reception) with coded mainly for information interaction and mail transmission using protocols。
V. Java mail api
The common api provided by java mail api java supports various e-mail protocols such as imap\pop3\smtp. Its core categories are summarized below:
1. Javax. Mail. Session
Defines the basic mail session, with all classes inheriting from the session and obtaining information from objects such as mail delivery servers, dispatch protocols, acceptance protocols, username passwords and other shared information throughout the application。
2. Javax. Mail. Store
Store is the access point to the mail account on the mail receiving server and can access the specified mail file via the setfolder method in the store class check
Three, javax. Mail. Folder
Folder represents mail folders for the analysis of organizational mail and access to mail in mail folders through the folder category
4 and javax. Mail. Message
The message class represents e-mail and contains mail messages, including address, mail title, date of dispatch, date of acceptance, mail content, etc。
5, javax. Mail. Address
Mail address, an abstract category, commonly subcategory javax. Mail. Internetaddress.
6 javax. Mail. Transport
Specifies the dispatch protocol (usually smtp) for mail to be sent through the specified mail server, which is also an abstract category, providing the send (message) static method for sending mail
Ssl transmission encryption
There are various insecurities in the chain of transmission on the internet: user information is intercepted during transmission, and the web site accessed by the user is an illegal site of personal online fraud。
Ssl is a mechanism to ensure secure communication between two nodes of the network, and ssl can be used to establish secure network connections. Network communication protocols, such as http and imap, use the ssl-based http as https, and use ssl encryption technology, when data are sent from one side to the other, the sender makes a face on the data and sends the secret text to the other. It is the encrypted data that is guaranteed to be transmitted on the network, and even if intercepted, it is not possible to obtain explicit information without a secret key. The data are declassified and processed by the recipient, the secure transmission of information between the parties and the confidentiality of the information。
