Someone at airport called me and said they have a cashier check for 750,000.00 for me but cannot go through custom. Is that correct?

First think. Do you know someone that would send you that kind of money and first spoke about it or some context. Usually if that amount of money is given from say an inheritance it is handled by an attorney. This has scam all over it.

It sounds like a scam

That sounds really fishy.. I would contact seatac security and start going through the appropriate channels to at least inform them, If nothing else.

If the money is coming internationally, then you are limited to $10K - federal law.

Sounds like a scam unless you know the person. They can't prevent you from taking a check through the airport