. Replaces all existing
 header values with this new value.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>header_name</code> - the name of this header</dd>
<dd><code>header_value</code> - the value for 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 
      