I wonder if you are setting your phone to delete mail from the server after downloading and that's why you don't see this elsewhere?
I don't think your Android mail client is inventing mail - I think that your other mail clients may be set to not show mail prior to a certain date or something like that.
I once had a similar spam, but after resetting my desktop mail to remove the spam filter, I scrolled all the way back in time - and I had the same older mail there as well.
If you're deleting mail on download, then perhaps try your web mail interface for roadrunner and see if the troublesome mail isn't still in your inbox, at the beginning.
So - I think the issue is we all need better spam filters, perhaps.