Internet Explorer 6 has problems with SquirrelMail and some charsets. This is a bug in the Win9x/ME IE6 language module.
The bug happens with ISO-8859-13 and ISO-8859-3 pages. If IE6 recieves it shows white page instead of page contents.
Workaround is to disable submision of such header in functions/i18n.php file.
This part of code in i18n.php causes problem with IE6
header( 'Content-Type: text/html; charset=' . $languages[$sm_notAlias]['CHARSET'] );
Installation of Openoffice.org v.1.1 fixes the problem. :)