The mail software must read in the JIS encoding and convert it to the local encoding, usually EUC on a Linux system. One of the things the mail software should check for is the Content-Type: definition of the incoming mail header. If it is defined as iso-2022-jp, the mail software should transparently do the appropriate conversions to allow the text to be displayed properly.
Content-Type: Text/Plain; charset=iso-2022-jp
The mail software should also add the Content-Type: tag to outgoing messages and identity Japanese mail as iso-2022-jp. Before sending, the mail software needs to convert the local encoding to iso-2022-jp.
Unfortunately, only a few software packages such as mew transparently handle the Content-Type: definition properly. Most of the e-mail sent of the Internet does not have a Content-Type: field. Also, mail sent as EUC or ShiftJIS without conversion may be received and sent properly by some mailers. This is due more to luck than to the robust nature of the mail software. Since the transmission of Japanese Internet mail as iso-2022-jp is such a widely accepted standard, it is better to conform to the standard instead of relying on luck.
The standardized and recommended method to send subject lines is to convert the subject line into Base64, the same encoding method used for MIME messages. In addition to encoding the subject line as Base64, the character encoding method should be defined and a series of ? delimiters are used to seperate the parts of a subject line.
A raw mail header looks like this:
Subject: =?iso-2022-jp?B?GyRCJDMkbCRPO2QkTiVGJTklSCRHJDkhIxsoQg==?= =?iso-2022-jp?B?IBskQkZ8S1w4bBsoQg==?=
The mail software scans the header and picks out the Subject:
keyword. Next, the
=? characters signal the beginning of the
iso-2022-jp identifies the encoding type of the
text of the subject line. This is usually the same as the body of
the message. However, it does not have to be. The next
character is a delimiter. The B identifies the encoding
method of the
iso-2022-jp encoded message as Base64. The
subject is encoded twice. First, the Japanese characters are
iso-2022-jp. Then the
is encoded with Base64. After the next
? the text of the subject
continues until the ending
?=. Long subjects are split into smaller
sections and each section contains the start and stop delimiters, character
encoding specification and transport encoding specification.