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Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the same time, in common with instant messaging. 

Today's email systems are based on a store-and-forward model. Email servers accept, forward, deliver and store messages.

Neither the users nor their computers are required to be online simultaneously; they need connect only briefly, typically to an email server, for as long as it takes to send or receive messages.

An email message consists of three components, the message envelope, the message header, and the message body. The message header contains control information, including, minimally, an originator's email address and one or more recipient addresses. Usually descriptive information is also added, such as a subject header field and a message submission date/time stamp.

Message format

The Internet email message format is now defined by RFC 5322, with multi-media content attachments being defined in RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions or MIME. RFC 5322 replaced the earlier RFC 2822 in 2008, and in turn RFC 2822 in 2001 replaced RFC 822 - which had been the standard for Internet email for nearly 20 years. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET.

Internet email messages consist of two major sections:
  • Header — Structured into fields such as From, To, CC, Subject, Date, and other information about the email.
  • Body — The basic content, as unstructured text; sometimes containing a signature block at the end. This is exactly the same as the body of a regular letter.
The header is separated from the body by a blank line.

Message header
Each message has exactly one header, which is structured into fields. Each field has a name and a value. RFC 5322 specifies the precise syntax.

Informally, each line of text in the header that begins with a printable character begins a separate field. The field name starts in the first character of the line and ends before the separator character ":". 

The separator is then followed by the field value (the "body" of the field). The value is continued onto subsequent lines if those lines have a space or tab as their first character. Field names and values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME encoded words.

Header fields
Email header fields can be multi-line, and each line must be at most 76 characters long. Header fields can only contain US-ASCII characters; for encoding characters in other sets, a syntax specified in RFC 2047 can be used. Recently the IETF EAI working group has defined some experimental extensions to allow Unicode characters to be used within the header. In particular, this allows email addresses to use non-ASCII characters. Such characters must only be used by servers that support these extensions.

The message header must include at least the following fields:
  • From: The email address, and optionally the name of the author(s). In many email clients not changeable except through changing account settings.
  • Date: The local time and date when the message was written. Like the From: field, many email clients fill this in automatically when sending. The recipient's client may then display the time in the format and time zone local to him/her.
The message header should include at least the following fields:
  • Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference in In-Reply-To.
  • In-Reply-To: Message-ID of the message that this is a reply to. Used to link related messages together. This field only applies for reply messages.

RFC 3864 describes registration procedures for message header fields at the IANA; it provides for permanent and provisional message header field names, including also fields defined for MIME, netnews, and http, and referencing relevant RFCs. Common header fields for email include:
  • To: The email address(es), and optionally name(s) of the message's recipient(s). Indicates primary recipients (multiple allowed), for secondary recipients see Cc: and Bcc: below.
  • 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 your inbox differently depending on whether you 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 mailinglist. Sendmail uses this header to affect prioritization of queued email, with "Precedence: special-delivery" messages delivered sooner. With modern high-bandwidth networks delivery priority is less of an issue than it once was. Microsoft Exchange respects a fine-grained automatic response suppression mechanism, the X-Auto-Response-Suppress header.
  • 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.


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