Are all Cashier Checks Valid? or can they be Fraudulent?

Answer 1

They can be fraudulent. Check out this site.
http://www.craigslist.org/about/scams.html
If you have reason to suspect the check you have may be fraudulent call or visit the issuing bank.

Answer 2

You often find on People's Court that a person is conned by a cashier cheque from overseas. Your bank will "cash" it for you as long as you have money in your bank. For example, someone sends you a $1000 cashier cheque. If you cash it at your bank, the assumption is that your account has that money. If the cheque proves to be invalid from the issuing bank days later, your bank will not credit you. It is no different than banks cashing any cheques by requiring several days to clear.

Answer 3

They can be fake.