der_name</code> - the name of this header</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="IllegalWriteException.html" title="class in jakarta.mail">IllegalWriteException</a></code> - if the underlying 
                                implementation does not support modification 
                                of existing values</dd>
<dd><code>java.lang.IllegalStateException</code> - if this Part is 
                                obtained from a READ_ONLY folder</dd>
<dd><code><a