in email, what is meant by multipart messaging?

Answer 1

A multipart email message has more than one section, each with its own header. The sections can be different parts of the message; for example, one section could be the text, and another could be a picture or other attachment. Or, the message could have multiple alternatives of the same content, like a plain text version and an html version, to allow the user's email client to choose which one to display.

Answer 2

Multipurpose Internet Mail Extensions (MIME) is an Internet e-mail standard that specifies how messages must be formatted so that they can contain content other than American Standard Code for Information Interchange (ASCII) printable characters. MIME enables the transfer of various types of files in an e-mail message. It also enables e-mail messages to be in other languages besides English. MIME also defines a format that describes e-mail messages that contain multiple parts, where each part can be any valid type of data. MIME is defined in RFCs 2045, 2046, 2047, 2184, 2048, 3023, and 2049. Simple Mail Transfer Protocol (SMTP) is a popular protocol used in the transfer of electronic mail messages over the Internet. SMTP supports the transmission of messages containing MIME formatted data between servers.

Each part of a MIME message contains data for one file or message encoded using only ASCII printable characters. A boundary, which occurs in the body as a delimiter between parts, is defined in the parameter "boundary" of the e-mail header "Content-Type". In the body of a multipart message, each part starts with two hyphen characters ("-"), followed by the defined boundary string, an optional linear whitespace, and a terminating CRLF. After its boundary delimiter line, each body part consists of a header area, a blank line, and a body area. The whole multipart message terminates with a closing boundary delimiter line, which is the boundary delimiter followed by two extra hyphen characters.

MIME multipart data contained within an SMTP message that is not terminated with a closing boundary delimiter line is a protocol anomaly. Some SMTP implementation may incorrectly process such unusual conditions. The security flaws caused by the receipt of malformed SMTP messages may be exploited for denial of service or other attacks.