Can you duplicate the problem by forwarding the spam by hand, as an attachment? If you can do that, then the plugin isn't at fault. I know, it's a hassle, but these kinds of problems don't get fixed unless someone is willing to put some time into isolating the problem.
I'm positive that it's a server problem, at least in my case, although it took quite a few test cases before DH support was convinced of that. I suspect, but am not 100% sure, that you're experiencing the same issue.
Here's a snippet from the most recent exchange. I wrote:
...I believe the problem stems from the attachment. If I had
to guess, I'd say that whatever filter you have that parses the
message header, looking for "to" and "from" addresses and rejecting
illegal or conflicting addresses, is mistakenly also checking the
header of the attachment and refusing the relay the message.
I am guessing that this filter does not get applied to mail sent
through the webmail interface.
Brian@DH replied (and I hope he doesn't mind my quoting his email here):
Yeah, that makes sense, our server checks for specific "from" and "to"
addresses, like from paypal.com or other known hosts that will NEVER be
part of our system.
Unfortunately, I don't have a quick fix for this, I'll need to have our
administration team look over the filtering rules and make the needed
changes. I apologize for the delay. For the time being, use webmail to
forward any spam you may get.
With the recent series of outages I'm sure this has been a low priority, but I'm still hopeful that it will get fixed eventually.