There's a phishing scam doing the rounds, especially on Yahoo!, that sends you a very official-looking email that says your account needs to be verified and will be deleted unless you verify it. It asks you to reply with your username, password, physical address, etc.
Rule number one and only: NO mail provider asks for your password. Not if it is a legitimate service provider. Yahoo!, Rediffmail, Gmail or other mail providers will never ask for your password.
I forwarded the one I got to the anti-phishing section of Yahoo! Customer Care (email@example.com) and received a reply, a snippet of which I quote below:
"Any unsolicited message asking for your Yahoo! ID and
password, security key, or other sensitive information is part of a scam to gain unauthorized access to your account."
So never give out your password, no matter how official the mail demanding it looks, even if it appears to come from your mail service provider. Check with your provider using the Help button and searching for "phishing", which is what this kind of scam is called.