most of the yahoo mail users I know are getting hacked and spamming links to all contacts.. looks like they are trying reduce the number of user requests they are getting (it would be pretty easy to create id's and request novel usernames multiple times).. Yahoo mail is the most insecure mail I have every used.
Available: https://edit.yahoo.com/reg_json?AccountID=mister_popular@&Pa...
Unavailable: https://edit.yahoo.com/reg_json?AccountID=test@&PartnerName=...
Step 1: Create site offering 10-email watch list for $1.99
Step 2: ...
Step 3: Profit?