These scams come masquerading as Paypal, ebay, citibank and dozens of others. It's really quite simple to avoid being fooled. Each and every one of these genuine entities hammer the same message over and over and over.......basically saying.....we will NEVER ask for ANY of your personal info (pin number, password, S.S. number, et al) in an email.
If you get one of these emails asking for any of this type of personal info, it is a scam.....plain and simple. If you are nervous that the warning of account problems may be true, contact them by phone or through their official website. As ferguson noted, many of these companies have spoof links for reporting suspicious email. If they don't, I open the official site of the entity and forward the scam email to them with a short explanation. My 2 cents