[Zope] weird occurrance with Zope MailTemplates

[Erik Myllymaki]

I am running into a strange problem with Zope MailTemplates; it probably 
reflects my rather shallow understanding of Zope in general.

I am using a Python Script as my index_html. It has a simple form and calls 
itself on submit.

And I use another Python Script that simply calls my MailTemplate object:

--index_html--

request = container.REQUEST
RESPONSE =  request.RESPONSE

if not request.has_key('next_state'):
    return container['index.zpt'](context, request)

if request['next_state']=="Client Lookup":
   error=''
   message=''
   try:
     container.send_template()
     message = "Mail Sent!"
   except:
     error = "Error sending Mail!"

   return container['index.zpt'](context, request, error=error, message=message)

--/index_html--

--send_template--
msg =  container.test_mt.as_message(
     mfrom='admin at mydomain.com'.encode('utf-8'),
     mto='ee at mydomain.com'.encode('utf-8'),
     subject='This is the Subject of a MailTemplate'.encode('utf-8'),
     message='This is the body'.encode('utf-8'),
     encoding='utf-8'
     )
msg.send()
return 'Mail Sent'
--/send_template--

My mailtemplate is the same on that is used in the examples(with a small typo 
corrected (s/option/options):

--test_mt--
       <tal:body xmlns:tal="http://xml.zope.org/namespaces/tal"
           xmlns:metal="http://xml.zope.org/namespaces/metal"
       >Dear <tal:x replace="options/mto"/>,

       <tal:x replace="user/getId"/> would like to thank you for
       your interest in:
       <a href=""
          tal:define="url root/absolute_url"
          tal:attributes="href url"
          tal:content="url"/>

       <tal:x replace="options/message"/>

       cheers,

       The Web Team
       </tal:body>

--/test_mt--

the send_template script works when tested from the ZMI.

When the MailTemplate was first created, it defaulted to text/plain. The weird 
thing is that this effected my main zpt in addition to the email. So after 
submitting the form, the zpt came back rendered as code instead of html, until I 
clued into what was happening and changed the MailTmeplate to text/html.

Is there a really good reason for this behaviour that I am missing?