AFAIK, some providers allow you to label, auto-filter, and make a folder for e-mails coming from a specific domain.
That doesn't help me when it comes to spam emails. It could be useful for labelling the domains that send the official newsletters, but I can't possibly know where the spam could originate from and make a label for that.
Have you considered doing this experiment with another member? So, using a shared e-mail for example.
No, not really. What difference would it make to the results if two or three people were involved in the experiment and shared the email addresses? Or maybe it's a thing of trust? If two people said there were no spam messages or there were spam messages, it's more believable than if just one person claims it.