Now I remember this guy! I remember asking him why he would not allow people to join if they had a Yahoo email address.
His response was "My problem has been that at least 50% of the time that I send an email to a yahoo user of my league the email is returned. I have never had this happen with any other pubic email address so I just figure might as well stay away from yahoo addresses"
I get it! He was striving for reliability in his league!
I've used a Yahoo email address since I began with online leagues and have never had the issues he was suggesting are commonplace 50% of the time. I remember dismissing the league as a serious one at that point.