MIME

Multipurpose Internet Mail Extensions (MIME) is an Internet Standard for the format of e-mail. Virtually all Internet e-mail is transmitted via SMTP in MIME format. Internet e-mail is so closely associated with the SMTP and MIME standards that it is sometimes called SMTP/MIME e-mail.

Encoded-Word

Since RFC2822 message header names and values are always ASCII characters, values that contain non-ASCII data must use the MIME encoded-word syntax (RFC 2047) instead of a literal string. This syntax uses a string of ASCII characters indicating both the original character encoding (the "charset") and the content-transfer-encoding used to map the bytes of the charset into ASCII characters.

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The form is: "=?charset?encoding?encoded text?=".

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• charset is often utf-8, but may be any character set registered with IANA. iso-2022-jp is common in Japan. iso-8859-1 and more recently iso-8859-15 are common in Europe.
• encoding can be either "Q" denoting quoted-printable encoding, or "B" denoting base64 encoding.
• encoded text is the quoted-printable or base64-encoded text.

For example,

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Subject: =?utf-8?Q?=C2=A1Hola,=20se=C3=B1or!?=

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is interpreted as "Subject: ¡Hola, señor!".

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The encoded-word format is not used for header names, such as Subject:. These header names are always in English in the raw message. When viewing a message with a non-English e-mail client, the header names are translated by the client.

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