I have a site, which all of a sudden is getting a lot of new users registering via the newsletter subscribe form on the site. The users emails are all from a range of free email address providers like oricoupon.buzz or streamboost.xyz etc. These sort of email address providers are know to be spammy and people use them for registering on sites, then the email address disappears, i.e. isn't a permanent email address, from what I understand.
Why do people bother with this? Is there a plugin I can use to stop this happening?