I can't use mailer taglib to send mails with GBK encoding. This forced me to 
send pure English mail to my customers who could only read Chinese mails.

I found the source code fragment:
 * $Header: /home/cvs/jakarta-
taglibs/mailer/src/org/apache/taglibs/mailer/MailTag.java,v 1.9 2002/10/31 
05:01:27 glenn Exp $

line 738~743:
    public final void setType(String value) {
	if (value.equalsIgnoreCase("html"))
	    type = "text/html";
	else
	    type = "text/plain";
    }

this restricted the message type only two options: "text/html", "text/plain"

But I do need "text/html;charset=GBK"!