Message format:
– [MIME header]
Content-Type: multipart/mixed
– [CONTENT]
HTML context
with alternativeTEXT (“with” HTML tags) file.
Contents are encoded using base64 algorithm
– [HTML part]
Content-Type: text/html
Content-Transfer-Encoding: utf-8;
– [TEXT file part]
Content-Type: text/plain
Content-Transfer-Encoding: utf-8;
HTML tags test:
(tag: strong): This is a test.
(tag: b): This is a test.
(tag: em): This is a test.
(tag: i): This is a test.
(tag: a href): This is a test.
(tag: font style, propties: red, bold): This is a test.
(tag: img): This is a test. 
UTF-8 characters test:
Chinese: 繁體中文測試 / 简体中文测试
Japanese: これはテストです
End of test.