About 20 years back, I worked at a decent sized midwest bank. a Cashier's check was a check drawn on the bank's own funds and signed by an officer of the bank. A certified check was a check drawn on a customer account which was guaranteed by the bank to be paid "when presented" ---- in perpetuity. This particular bank physically altered the customer check so it could not be processed through his or her account. The funds were then withdrawn and transferred to a trust account reserved for payment of certified checks and the check was logged. The customer then signed an acknowledgement that only the certified check could be used to receive the funds set aside, and that in the event that the check was claimed to be lost or destroyed the bank had no duty to refund the check or reissue it unless an irrevocable bond for the full amount of the lost or destroyed check was obtained. This bank did not, under any circumstances, certify its own cashiers checks, only checks drawn on customer accounts Hope this helps.
I concur on the opinions above that someone mentioning or promising a certified cashier's check has a high probability of being a scammer.