go で utf8メールを送信 ref: http://qiita.com/yamasaki-masahide/items/a9f8b43eeeaddbfb6b44

file1.txt

Subject: =?utf-8?B?6ZW344GE44K144OW44K444Kn44Kv44OI6ZW344GE44K144OW44K4?=
 =?utf-8?B?44Kn44Kv44OI6ZW344GE44K144OW44K444Kn44Kv44OI6ZW344GE?=
 =?utf-8?B?44K144OW44K444Kn44Kv44OI6ZW344GE44K144OW44K444Kn44Kv?=
 =?utf-8?B?44OI6ZW344GE44K144OW44K444Kn44Kv44OI6ZW344GE44K144OW?=
 =?utf-8?B?44K444Kn44Kv44OI?=
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
Message-Id: <20140222115231.8DECE260593@st>
Date: Sat, 22 Feb 2014 20:52:31 +0900 (JST)

5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys
5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paH5pys5paHCuacrOaW
h+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+ac
rOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWh+acrOaWhwo=

go-utf8-sendmail-example.go

package main

import (
    "bytes"
    "encoding/base64"
    "log"
    "net/mail"
    "net/smtp"
    "strings"
)

// 76バイト毎にCRLFを挿入する
func add76crlf(msg string) string {
    var buffer bytes.Buffer
    for k, c := range strings.Split(msg, "") {
        buffer.WriteString(c)
        if k%76 == 75 {
            buffer.WriteString("\r\n")
        }
    }
    return buffer.String()
}

// UTF8文字列を指定文字数で分割
func utf8Split(utf8string string, length int) []string {
    resultString := []string{}
    var buffer bytes.Buffer
    for k, c := range strings.Split(utf8string, "") {
        buffer.WriteString(c)
        if k%length == length-1 {
            resultString = append(resultString, buffer.String())
            buffer.Reset()
        }
    }
    if buffer.Len() > 0 {
        resultString = append(resultString, buffer.String())
    }
    return resultString
}

// サブジェクトをMIMEエンコードする
func encodeSubject(subject string) string {
    var buffer bytes.Buffer
    buffer.WriteString("Subject:")
    for _, line := range utf8Split(subject, 13) {
        buffer.WriteString(" =?utf-8?B?")
        buffer.WriteString(base64.StdEncoding.EncodeToString([]byte(line)))
        buffer.WriteString("?=\r\n")
    }
    return buffer.String()
}

func main() {


    from := mail.Address{"ふろむ", "from@hoge.fuga"}
    to := mail.Address{"とぅー", "to@fuga.hoge"}
    title := "長いサブジェクト長いサブジェクト長いサブジェクト長いサブジェクト長いサブジェクト長いサブジェクト長いサブジェクト"

    var body bytes.Buffer
    body.WriteString("本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文\n")
    body.WriteString("本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文本文\n")

    var header bytes.Buffer
    header.WriteString("From: " + from.String() + "\r\n")
    header.WriteString("To: " + to.String() + "\r\n")
    header.WriteString(encodeSubject(title))
    header.WriteString("MIME-Version: 1.0\r\n")
    header.WriteString("Content-Type: text/plain; charset=\"utf-8\"\r\n")
    header.WriteString("Content-Transfer-Encoding: base64\r\n")

    var message bytes.Buffer
    message = header
    message.WriteString("\r\n")
    message.WriteString(add76crlf(base64.StdEncoding.EncodeToString(body.Bytes())))

    err := smtp.SendMail(
        "localhost:25",
        nil,
        from.Address,
        []string{to.Address},
        []byte(message.String()),
    )
    if err != nil {
        log.Fatal(err)
    }
}