I was having the same problem with the default 1.0b install. I changed from using the mail() function to specifying the SMTP server info in the config, and that seemed to fix the problem. I think it's related to the issue patched with the config changes noted on the 
<a href="http://roundcube.com">roundcube.com</a> homepage news items. <br><br>
<div><span class="gmail_quote">On 3/15/06, <b class="gmail_sendername">Paul</b> &lt;<a href="mailto:paul@webspectrum.co.uk">paul@webspectrum.co.uk</a>&gt; wrote:</span>
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<p><font face="Arial" size="2"><span>I'm getting what looks like header type info at the top of the body of emails I send from using Roundcube, can I turn all this off so it doesn't appear ??</span></font></p>
<p><font face="Arial" size="2"><span>See below example :</span></font></p>
<p><font size="+0">Date: Thu, 16 Mar 2006 0:01:55 +0000 From: <a onclick="return top.js.OpenExtLink(window,event,this)" href="mailto:info@domain.co.uk" target="_blank"><font color="#000000">info@<span>domain</span>co.uk
</font></a><span>&nbsp;</span></font></p>
<p><font face="Arial" size="2">Message-ID: &lt;24998d0f04ce987302a1342d92520d1f@localhost&gt; X-Sender: <a onclick="return top.js.OpenExtLink(window,event,this)" href="mailto:info@domain.co.uk" target="_blank">info@<span>
domain</span>.co.uk</a> Content-Type: text/plain; charset=&quot;UTF-8&quot; Content-Transfer-Encoding: 8bit </font></p></div>
<div><font face="Arial" size="2"></font>&nbsp;</div></div></blockquote></div><br><br clear="all"><br>-- <br>Dan Schaper