Does JavaMail support Multipart nesting?

It is possible to access all the parts of a message, even if they are nested multiparts. One possible approach is to work recursively and "flatten" out the parts into one straight list (this is the one I prefer, even if it does not reflect exactly how the message looks like "architecturally").
You start from:

[...]
if(msg.isMimeType("multipart/*")) {
  //get main body part
  Multipart mp=(Multipart)msg.getContent();
					
  //build flatlist					
  List partlist=new ArrayList(10);	//example, use your own size idea
  buildPartInfoList(partlist,mp);
}
[...]

[...]
private void buildPartInfoList(List partlist, Multipart mp) 
    throws Exception {
	
  for (int i=0; i<mp.getCount(); i++) {
    //Get part
    Part apart=mp.getBodyPart(i);
    //handle single & multiparts
    if(apart.isMimeType("multipart/*")) {
      //recurse
      buildPartInfoList(partlist,(Multipart)apart.getContent());	
    } else {
      //append the part
      partlist.add(apart);
    }
  }
}//buildPartList
[...]

If you do not like this approach, you can easily extract the idea of how to retrieve a specific nested part from the code above.