add footer/signature to mail in Java

Question

In one of my Java applications I have to forward e-mails. So, I get e-mails (plain text or multipart) with any content (maybe also attachments). I edit their subject, from- and to-header and send them via SMTP.

I already implemented this using Apache James Mime4j and Apache Commons Net but now I also have to append a footer/signature to the content of each e-mail.

Can I achieve this with Mime4j too? Would be great! How? If not: is there another way?

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EDIT: Details after Wolfgang Fahl's comment:

Maybe I'm using the library the wrong way, but I am parsing the message as follows:

MessageBuilder messageBuilder = new DefaultMessageBuilder();
InputStream in = ...
Message m = messageBuilder.parseMessage(in);

Now I have an instance of Message and I can set it's subject, sender, etc. But Message does not provide a method setText() or something like that. Ok, there is getBody() but then I don't know how to manipulate the Body.