I have two email addresses on yahoo. One I get most of my mail and the other is a recent address for when I need an email address for a product registration. I also have a subscription to mlb.com (major league baseball) where I get emails from their site to the older yahoo email address. A few days after I started the new yahoo address I started getting emails there from mlb.com stating that I had signed up for them (which I had not done at the new email address). I only get 2-3 emails per week at the new email address, ones from mlb.com being most of them. How did mlb.com get my new email address? Did yahoo sell it to them? I have their site bookmarked in my favorites. Could their cookie have somehow figured out my new address? I have not been signed in at mlb.com in months. MLB.com uses doubleclick.net for ads and they also promote a toolbar powered by ask.com. So I am little leery of their site.
I wouldn't discount any connection between Yahoo and MLB. I noticed that when I go to certain websites (such as online banking sites), my Yahoo opening page would display ads for online banks accounts. I know that yahoo tailors a surfer's welcome page to whatever sites that surfer tends to go to (I am sure they are tracking you). E-commerce and luring potential customers to other sites involves commissions and big $. As for email address sharing, that I am unsure of since I haven't noticed that particular situation that you mention.
If you added your new email address to your Yahoo acct., rather than just opening a new email address there, they may be sharing your contact list. Check that and also you should read Yahoo's Terms of Service and Privacy Statements.
I suspect all of the senerios mentioned in this thread are possible and no doubt happening. The advice to check ToS on Yahoo is solid. My ISP using Yahoo offers multiple email account for up to x family members. Also they provide what they call Address Guard, Block address and Spam protection. I won't try to describe all those unless you want me, too but the address guard allows you to create disposible email addresses for use on forums, MLB sites, car rentals etc. Then if/when you get spam or they sell it and then you get spam you simply delete it and the spam is gone! They even sent me an email saying they noticed I was getting a lot of spam on one of the disposables and suggested I delete it! Hope this helps.
