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E-mail is still the most used Internet app. According to some studies around 85% of Internet users still use e-mail for communication. Electronic mail is a method to exchange digital messages from a sender to one or more recipients. It is based on a store-and-forward model in which neither the users nor their computers need to be online simultaneously. They connect briefly to a mail server to send or receive messages. E-mail servers accept, forward, deliver, and store messages. It uses addresses composed of name@domain. 2
The Internet e-mail application is based on Simple Mail Transfer Protocol or SMTP. With this protocol the sender s Mail User Agent (MUA) formats the message into an e-mail format and submits it to the local mail submission agent (MSA that is commonly known as mail server). The MSA determines the destination address provided in the SMTP protocol (that, as we will see, is not the one from the message header), and resolves the domain name to determine the name of the mail server for this domain. It gets it from the DNS server of the destination domain, that responds with an MX record that contains the mail exchange servers for that domain (that are called a Message Transfer Agent MTA). It then sends the message to the first MTA. Sometimes this MTA is the final Message Delivery Agent (MDA) and sometimes the message has to be forwarded to other MTAs before it reaches the MDA. The MDA delivers the message to the mailbox of the recipient user, that can receive it using one of several protocols: Post Office Protocol (POP3), Internet Message Access Protocol (IMAP) or a webmail session with the server. 3
An Internet email message consists of three components, the message envelope, the message header, and the message body. The message envelope is used by the SMTP protocol and contains the information that will be used for routing the message; it is not shown to the recipient. The message header contains control information intended to be used by the recipient, including, minimally, an originator s email address and one or more recipient addresses. It usually contains other fields such as a subject header field and a message submission date/time stamp. Email was originally an ASCII text-only communications medium (even today most email clients still give us the possibility of sending a text only message), but this was not enough and it was extended by what is called Multipurpose Internet Mail Extensions (MIME) that allows us to send text in other character sets using html formatted content and add multi-media content attachments. Usually email clients send messages with 2 versions included, one in plain text and the other formatted in html. To support for non ASCII characters used in non English languages, internationalized email addresses using UTF-8, have been standardized, but they are not yet widely 4
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Here we can see some of the fields that are included in the header of an email message. These fields are used by the recipient, as the addresses used to send the message are the ones included in the SMTP envelope. Subject: A brief summary of the topic of the message. Certain abbreviations are commonly used in the subject, including "RE:" and "FW:". Bcc: Blind carbon copy; addresses added to the SMTP delivery list but not (usually) listed in the message data, remaining invisible to other recipients. Cc: Carbon copy; many email clients will mark email in one's inbox differently depending on whether they are in the To: or Cc: list. Content-Type: Information about how the message is to be displayed, usually a MIME type. Precedence: commonly with values "bulk", "junk", or "list"; used to indicate that automated "vacation" or "out of office" responses should not be returned for this mail, e.g. to prevent vacation notices from being sent to all other subscribers of a mailing list. Sendmail uses this header to affect prioritization of queued email, with "Precedence: special-delivery" messages delivered sooner. With modern highbandwidth networks delivery priority is less of an issue than it once was. Microsoft Exchange respects a fine-grained automatic response suppression mechanism, the X- 5
Auto-Response-Suppress header.[70] References: Message-ID of the message that this is a reply to, and the message-id of the message the previous reply was a reply to, etc. Reply-To: Address that should be used to reply to the message. Sender: Address of the actual sender acting on behalf of the author listed in the From: field (secretary, list manager, etc.). Archived-At: A direct link to the archived form of an individual email message. SMTP defines the trace information of a message, which is also saved in the header using the following two fields: Received: when an SMTP server accepts a message it inserts this trace record at the top of the header (last to first). Return-Path: when the delivery SMTP server makes the final delivery of a message, it inserts this field at the top of the header. 5
To receive the messages we have two options: installing an e-mail client application or, if our e-mail server gives us the option, using a webmail interface that enables us to read and manage the emails from the server. If we use a client application, we will have to configure it to use one of the two protocols available for clients in Internet mail, POP3 or IMAP. With POP3 the client connects to the server and downloads all the new messages, erasing them from the server and storing them locally. With IMAP the messages are stored on the server and the client reads them from it. You can create folders on the server to organize your stored emails. The advantage of IMAP and webmail over POP3 is that, as they are stored on the server, you can read your stored messages from any computer while with POP3 you can only read them from the computer where they are stored. If you have limited storage capacity on the server you may need to use POP3 to avoid running out of space. POP3 and IMAP send the messages unencrypted, so they can be read by anyone that intercepts them. Most servers give you the possibility of adding encryption security to 6
both protocols (for webmail the server should provide you with HTTPS access). Some common email client apps are Mozilla Thunderbird, the old Outlook Express, that was included with windows up to XP version, em Client or Mail, the one included in Mac OS. You can also configure enterprise mail clients such as Microsoft Outook (included in Office) or IBM Lotus Notes to work with Internet mail accounts, and if you are using a corporate account, with an Exchange Microsoft server or a Lotus Notes Server for example, your company will have a gateway that lets you communicate seamlessly with Internet mail. And every mobile Operating System has its email client. E-mail clients have features to handle conversations with the same subject, mark messages with colors or ask for read confirmation. Most of them will read RSS feeds for you and present them as e-mails. There are several free email providers whose main interface is webmail. The biggest ones are Gmail from Google and Outlook.com from Microsoft. Most of them can be configured to be accessed with POP3 or IMAP. 6
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