Ouch...
That's a little harsh...
Unfortunately I can't post any source code due to 2 reasons.
1. I'm using proprietary api's that deal with encryption and certificates
that are written by a company called Entrust.
2. The api's are used to connect to a secure server for a client that I'm
not a liberty to divulge.

That being said, I apologize for the ambiguity, but I'm limited on what I
can say.

So let's try this again.

I need to be able to construct a secure multipart mime message with custom
headers.
I need to be able to sign my  multipart parts with a secure signature
I need to be able to modify the multipart part headers to remove and add
specific header lines.
I need to encrypt the multipart parts (again) using pk7 encryption and
stream the encrypted object to an internet server.

I've completed the above tasks with the exception of the 3rd item, modifying
the multipart headers.
The only way I've found to create a header object is to use InternetHeaders
from javax.mail.internet.
I can see the header lines, but I can't get them out of the Enumeration
object.

eg.        InternetHeaders hedr = new InternetHeaders();
           Vector headlines;
           Enumeration henum =  hedr.getAllHeaderLines();
           while(henum.hasMoreElements())
           {
               headlines.add(henum.nextElement())
           }
That's what blows up.
(well not exactly, it just returns null)

So if you have any advice on how to manipulate message headers or know of
any resources out there that I can read to educate myself, I would greatly
appreciate it.
If anyone has done development with the Entrust encryption software that
uses posting of s/mime messages to an internet server and has some tips to
share, please contact me via my gmail address.

and yes... I've googled the snot out of this one.