Email should be encoded with UTF8 and base64

[miyauchi]

The plugin WP Multibyte Patch included in the Japanese package is changing the Email charset from UTF-8 to ISO-2022-JP.

​https://ja.wordpress.org/plugins/wp-multibyte-patch/

Almost all Email clients in Japan can handle UTF-8 now, so I am believing that we don't need to change email charset to ISO-2022-JP.

Also, if we activate WP Multibyte Patch and switch language to another language like FR_fr, the character in the email body will be broken.

But I guess email should be encoded by base64 for non-ascii users. I know 8bit encoding works good, but I guess base64 is reliable for us if we remove the plugin from Japanese package.

Following is an example email header of the Gmail. Gmail is UTF8 + base64.

Subject: =?UTF-8?B?xxxxxxxxxxxxxxxxxx==?=
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: base64

Related: ​#meta3163, #43829, #44296, #44541, #15955, #22363

Following is a list of tickets to improve multibyte enhancement.

​https://docs.google.com/spreadsheets/d/13oGbc7AqEN6OUvmze-JDCKXDdruFqsxqSAdI7-b6Lho/edit#gid=0

[ocean90]

It would be great to see how such a broken email looks like with the raw headers. What's the value of get_bloginfo( 'charset' )?