mail: modifing a body

[rabarama]

I've a multipart mail, i parse it creating a Tmail object.
In the body i have "---start---" "---end---" marks that indicates a part of
the text to delete.
If i read the email and then deliver it without touching the body, it sends
the correct mail (html format, attachments and so on)
If i do someting like:
mail.body=mail.body[0,mail.index("---start---")]+mail.body[mail.body.index("---end---",mail.body.length]
when i send that mail i lost every html format and multipart data.

How modify the body without loosing the rest?