it does seem strange that you can send bulk via your web mail access, but not with WLM.
I do know web mail and email client software are handled differently, so it may still be worthwhile talking to your ISP, if you can make them understand the problem.
I just had a thought. In WLM, do you select BBC for the first recipient in your list marked or is it a "To", (eg not hidden)? If BCC, try it with "To" for the first one. Perhaps the first one can be your own email address, and the 2nd one any other email address you have, but marked as BCC, as a test, so that when you get both back you can see if the first is hidden from the 2nd.
Sorry, I can see that looks complicated. I will try and explain if you wish.
What about another test? Download and install Mozilla's Thunderbird email software, set up your ISP email account in that, then see if you can send bulk emails in TB. Both TB and WLM will live with each other quite comfortably, (like different browsers), but since this would be a test, make sure all your emails have been downloaded into WLM first before installing and opening this software. Then if you don't like it or it doesn't work, you can uninstall it without losing any emails.