<message>
<header>
<field>
Content-Type: multipart/mixed; boundary="boundary"</field>
</header>
<multipart>
<preamble>
</preamble>
<body-part>
<header>
<field>
Content-Type: text/plain</field>
</header>
<body>
first part
</body>
</body-part>
<epilogue>

From the RFC about ending boundary:

===================================================================
NOTE TO IMPLEMENTORS:  Boundary string comparisons must compare the
boundary value with the beginning of each candidate line.  An exact
match of the entire candidate line is not required; it is sufficient
that the boundary appear in its entirety following the CRLF.
===================================================================

--boundary--

The above boundary should be part of the epilogue, too.</epilogue>
</multipart>
</message>