One solution is using a FORM with an ACTION of POST instead. The caveat is that FORM elements cannot be used to fill in the various fields like Subject, Body, and and so forth because the mailto: protocol handler does not examine these fields. All the FORM data is packaged and e-mailed away.
So, having the FORM ACTION fills in the Subject field as follows
<FORM Action="mailto:xyz?Subject=Test_Post" METHOD="POST">
mailto: protocol test:
<Br>Subject:
<INPUT name="Subject" value="Test Subject">
<Br>Body: 
<TEXTAREA name="Body">
kfdskfdksfkds
</TEXTAREA>
<BR>
<INPUT type="submit" value="Submit">
</FORM>
while having the FORM ACTION illustrated here does not:
<FORM Action="mailto:xyz" METHOD="POST">
mailto: protocol test:
<Br>Subject:
<INPUT name="Subject" value="Test Subject">
<Br>Body: 
<TEXTAREA name="Body">
kfdskfdksfkds
</TEXTAREA>
<BR>
<INPUT type="submit" value="Submit">
</FORM>
In both cases, the FORM data is e-mailed in as an Attachment, in an encoded format. For instance, in the preceding case, this is how the data looks:
Subject=Test+Subject&Body=%09kfdskfdksfkds%0D%0A%09
This is because the default ENCTYPE attribute for the FORM element is "application/x-www-form-urlencoded". To e-mail data in plain-text format instead, explicitly specify an ENCTYPE attribute of "text/plain". For instance:
<FORM Action="mailto:xyz" METHOD="POST" ENCTYPE="text/plain">
mailto: protocol test:
<Br>Subject:
<INPUT name="Subject" value="Test Subject">
<Br>Body: 
<TEXTAREA name="Body">
kfdskfdksfkds
</TEXTAREA>
<BR>
<INPUT type="submit" value="Submit">
</FORM>
produces the following Body:
Subject=Test Subject
Body= kfdskfdksfkds