Lots of quibbles and maybe they will make a difference, but I don't think that will last long and here is the reason.

First off any email addy thats got privilidges in my accounts does not get any check at all, ie I always get emails from people I have added.
Secondly any other casual email is usualy off of a site where I have privilidges and will be so labeled. No sweat, the rest is unsolicited and therefore unwelcome, anything else if that important would be followed up if not already preceeded by a phone call.

Third and most important of all, - John in Kansas almost lets one imagine that it does not matter ..but it does! - today most smart netizens are using webmail precisely because it is so well financed and kept uptodate with virus/hazard filters, and todays servers are sharing across boundaries profiles for such data as seems appropriate according to user and/or their own moderation arrangements... ahem..

In addition, some of the bigger players are now sharing data and guess what? most spam most of the time never gets into users email boxes.
In effect when so many users flag a package as spam that the server responds, the item ceases to exist.. squish splat gone.

Email lists continue to work just fine if you go to the trouble of setting it up properly. In fact, Yahoo, for example, provides you the netizen with a free service.

If you want to encourage these spammers scumbags by returning their rubbish or by localy deleting - BTW which LEGALLY alerts them that you have recieved their crap - then go ahead. But please be aware that 50% of the bandwidth we are paying for is spam.

Simply chosing a webmail solution /ignore and trash spam/ is closing it down faster than the individual actions which have been tried and failed for the last 10 years.