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An e mail client is a program which is used to send, receive the mails. It is working as mail user agent for Email server to manage email. A mail user agent functions by connecting to a mailbox into which e-mail has been fetched and stored in a particular format. It typically presents a simple user interface to perform tasks with the mail. A mail client by itself is incapable of sending or retrieving mail. The use of a mail program such as Outlook, Eudora or Thunderbird is the traditional approach. The advantage of such programs is that they are rich in features compared with Web-based mail and are generally preferred. Their disadvantage is that your e-mail is tied to the machine the software is installed in. To use another computer, you have to install the mail client and configure the ISP's server settings all over again Email Client Program Work in following steps. Message Receiving Sending Message Formatting Message Encryption Message Sending & Receiving Like most client programs, a email client program is only active when a user runs it. Messages arrive on the Mail Transfer Agent (MTA) server. Unless the email client program has access to the server's disk, messages are stored on a remote server and the email client program has to request them on behalf of the user. In the first case, shared disk, a user logs on a server and runs a email client program on that machine. The email client program reads messages from a conventionally formatted storage, typically mbox, within the user's HOME directory. The MTA uses a suitable Mail Delivery Agent (MDA) to add messages to that storage, possibly in concurrence with the email client program. This is the default setting on many Unix systems. Webmail applications running on the relevant server can also benefit from direct disk access to the mail storage. For personal computing, and whenever messages are stored on a remote system, a mail user agent connects to a remote mailbox to retrieve messages. Access to remote mailboxes comes in two flavors. On the one hand, the Post Office Protocol (POP) allows the client to download messages one at a time and only delete them from the server after they have been successfully saved on local storage. It is possible to leave messages on the server in order to let another client download them. However, there is no provision for flagging a specific message as seen, answered, or forwarded, thus POP is not convenient for users who access the same mail from different machines or clients. On the other hand, the Internet Message Access Protocol (IMAP) allows users to keep messages on the server, flagging them as appropriate. IMAP provides sub-folders. Typically, the Sent, Drafts, and Trash folders are created by default. Both POP and IMAP clients can be configured to access more mailboxes at the same time, as well as to check each mailbox every given number of minutes. IMAP features an idle extension for real time updates, which may work better than polling when long lasting connections are feasible. Message Formatting The email client programs usually have built-in the ability to display and edit text. Editing HTML text is a popular feature. Invoking an external editor may be an alternative. The Email Client Program responsibilities include proper formatting for headers and body, and MIME for non-textual content and attachments. Headers include the destination fields, To, Cc, and Bcc, and the originator fields From which is the message's author, Sender in case there are more authors, and Reply-To in case responses should be addressed to a different mailbox. To better assist the user with destination fields, many clients maintain one or more address books and/or are able to connect to an LDAP directory server. For originator fields, clients may support different identities Message Encryption Email encryption refers to encryption, and often authentication, of e-mail messages. E-mail encryption usually relies on public-key cryptography. E-mail encryption enables to safeguard privacy by encrypting the mail sessions, the body of the message, or both. The Message Encryption divided in two steps. 1) Encryption of Mail Session 2) Encryption of Mail body 1) Encrypted mail sessions Encrypted mail sessions deliver messages in their original format, i.e. plain text or encrypted body, on a user's local mailbox and on the destination server's. The latter server is operated by an email hosting service provider, possibly a different entity than the internet access provider currently at hand. 2) Encryption of Mail body There are two models for managing cryptographic keys. S/MIME employs a model based on a trusted certificate authority that signs users' public keys. OpenPGP employs a somewhat more flexible web of trust mechanism that allows users to sign one another's public keys. OpenPGP is also more flexible in the format of the messages, in that it still supports plain message encryption and signing as they used to work before MIME standardization.
Article Source: http://www.phalenes.org/articles
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