<message>
<header>
<field>
Content-Type: multipart/mixed; boundary="outer-boundary"</field>
</header>
<multipart>
<preamble>
Outer preamble
</preamble>
<body-part>
<header>
<field>
Content-Type: text/plain</field>
</header>
<body>
Foo
</body>
</body-part>
<body-part>
<header>
<field>
Content-Type: multipart/alternative; boundary="inner-boundary"</field>
</header>
<multipart>
<preamble>
AAA
</preamble>
<body-part>
<header>
</header>
<body>
</body>
</body-part>
<epilogue>
</epilogue>
</multipart>
</body-part>
<epilogue>
Outer epilouge
</epilogue>
</multipart>
</message>