Email format:

Header and body

The standard format for an e-mail is laid down here 'RFC 5322', and is referred to as the Internet Message Format (IMF). Simple put, the format separates an e-mail into two components,

1. Header
2. Body

MIME is an important supplement to 'RFC 5322'. Which allows things such as non-textual attachments in e-mail bodies and multipart messages. SMTP does not require MIME to transfer an e-mail, nor POP to receive an e-mail. But, in essence all e-mail's now come MIME formatted, due to the extensive non-textural features it provides.

Header

The header summerises the sender and receiver of the e-mail. Each e-mail will only contain one header. A header has many field segments within it. Below are some of the fields found within the header,

Primary fields

• From: The sender's address.
  
• To: The recipients address, that can include more than one address.
  
• Subject: Title of the message, gives an indication of the contents of the e-mail.
  
• Date: Time the message was sent, typically included by the mail service.
  
• Message-ID: Also generated by the mail service. Makes sure one message is sent, and whether it's a reply.

Secondary fields

• Bcc (Blind Carbon Copy): For multiple addresses, denotes who should not read the other recipients.
  
• Cc (Carbon copy): For multiple addresses, denotes who can read the other recipients.
  
• Content-Type: Provides the text and character format.
  
• Importance: Will indicate whether the message is urgent or normal.
  
• In-Reply-To: Denotes whether the e-mail has been to replied to previously.
  
• Precedence: Indicates which mail folder the e-mail will go into, such as 'spam' or 'junk'.
  
• Received: Lists all the servers which have handled the message.
  
• Return-Path:: Email address which a reply will be sent to.
  
• Sender: Can display the sender ID, if acting on the orders of the address owner, secretary for boss etc.
  
• X-Originating-IP: Will show the sender IP address, which can be either static or dynamic.

MIME fields

• MIME format: Shows the the MIME format. Such as 'MIME-Version: 1.0'.
• Content encoding: Defines if binary data has been represented in ASCII text format.
  
• Content type: The standard is text/plain, but multipart/mixed is an option.
• Content-Disposition: Allows for attachments, such as binary files.
  

For a full list of fields.

Body

After the content encoding and type are specified, the message body is where the content is provided.

The SMTP protocol transfers e-mail's from one computer to another, but is only capable of supporting 7-bit ASCII characters. Which is of little use if you wish to write e-mail's in languages other than the Latin alphabet, or attach files to an e-mail. As highlighted above, the MIME header allow e-mail's to provide MIME content.

MIME is an Internet standard which allows e-mail to provide additional character sets (not just ASCII), binary attachments, multiple body parts and header fields not in ASCII. If an e-mail is formatted in HTML, a plain text version is usually sent as well, incase the receiving e-mail server does not support MIME extensions.