Wow, you must be really frustrated!
On the theory that it's not coming thru a server, I don't know of any malware that could be spoofing that - but obviously, I don't know everything. I might suggest a quick scan of your phone with Lookout, or Avast or the like, just to be safe, you can get those free in the Market and uninstall them if you don't like them.
What we really need here is access to the raw mail headers for these offensive messages. Few Android mail clients display that, and for those that do, the feature seems to come and go, from what I've just read. In your list of mail messages, try a long press and see if you can get "details" or anything like that.
If not, I'll track down a mail client that will allow you to see that info - if one exists, I'll find it.
Please help me out with some info - what's your phone model and what email client are you using now?
Oh, and sorry about the ads - we're at about a million members, they help keep the lights on.