That is a scammer trying to hi-jack your email address to spam all your contacts and then use the account to spam hundreds/thousands of others.
Yahoo and all email companies, all banks and all companies in the entire world will NEVER ask for your password, pin or date of birth. No Exceptions Ever.
Ignore and delete that email and any others demanding such information.
If you have responded to a scammer, you are on his 'potential sucker' list, he will try again to separate you from your cash. He will send you more emails from his other free email addresses using another of his fake names with all kinds of stories of needing your password, great jobs, lottery winnings, millions in the bank and desperate, lonely, sexy singles. He will sell your email address to all his scamming buddies who will also send you dozens of fake emails all with the exact same goal, you sending them your cash via Western Union or moneygram.
Do you know how to check the header of a received email? If not, you could google for information. Being able to read the header to determine the geographic location an email originated from will help you weed out the most obvious scams and scammers. Then delete and block that scammer. Don't bother to tell him that you know he is a scammer, it isn't worth your effort. He has one job in life, convincing victims to send him their hard-earned cash.
Whenever suspicious or just plain curious, google everything, website addresses, names used, companies mentioned, phone numbers given, all email addresses, even sentences from the emails as you might be unpleasantly surprised at what you find already posted online. You can also post/ask here and every scam-warner-anti-fraud-busting site you can find before taking a chance and losing money, email address or identity to a scammer.
If you google "yahoo email phishing scam", "email hijacked viagara porn spammer" or something similar you will find hundreds of posts of victims and near victims of this type of scam.
In fact, if you check out the section here at Yahoo Answers entitled "Yahoo email, spam and bulk mail" you will find hundreds of questions from victims who have had their email address hi-jacked or spoofed by scammers sending out porn spam, send money scams and links to fake websites pretending to sell name brand merchandise.