Thanks .m.!
Quite strange that my messagebit looks a little different:
<tr>
<td align="center" class="trow1" width="1%"><img src="{$theme['imgdir']}/{$msgfolder}" alt="{$msgalt}" title="{$msgalt}" /></td>
<td align="center" class="trow2" width="1%">{$icon}</td>
<td class="trow1" width="35%">{$msgprefix}<a href="private.php?action=read&pmid={$message['pmid']}">{$message['subject']}</a>{$msgsuffix}{$denyreceipt}</td>
<td align="center" class="trow2">{$tofromusername}</td>
<td class="trow1" align="right" style="white-space: nowrap"><span class="smalltext">{$senddate}</span></td>
<td class="trow2" align="center"><input type="checkbox" class="checkbox" name="check[{$message['pmid']}]" value="1" /></td>
</tr>
But i've edited the first td line with the one you have provided and it seems to work.