said by switcher7:Apparently Yahoo plus is not necessary, or I mis-understood what I was reading.
Thanks again for your help.
I believe it is related to allowing mobile devices to access email. I am guessing that they can't reliably block free PC access while allowing free mobile device access.
There is also the issue of value added goodies, such as AddressGuard. I have a free, 'att.net' account signed up on 07/23/2010 which has AddressGuard. But another signed up on 10/26/2011 does not.
AddressGuard is available to Yahoo! Mail Plus, and co-branded ISP accounts; I still have it with my 2001-issue 'pacbell.net' accounts. AddressGuard is not available to free Yahoo! Mail accounts, or current free 'att.net' accounts. But the cutoff date seems later than 2006, based on my signup dates.