Mail: modifing a body

I’ve a multipart mail, i parse it creating a Tmail object.
In the body i have “—start—” “—end—” marks that indicates a part
the text to delete.
If i read the email and then deliver it without touching the body, it
the correct mail (html format, attachments and so on)
If i do someting like:
when i send that mail i lost every html format and multipart data.

How modify the body without loosing the rest?