What will happen if I send a mail without a stamp?

If you remembered to put a return address on it, then it should be returned to you.

Taxes often require extra postage because of weight or thickness.

it will be returned to you if u provided a return address. you might get lucky and the postman pay if for you as well . if u didnt put a return adress it will eventually be destroyed

It mmight be returned to you. But it won't be delived to the IRS. Send another copy.

You will be arrested and ultimately executed.

It will be returned to you, as long as there is a return address.

It will come back to you if you put your address on it

It will be returned to you.

What happens in England is, it's still sent just the same but then the receiver is asked if they want to pay for it or refuse it, but a lot of the time they just post the letter anyway. I've got letters without stamps before and they had an ink stamp on the front saying "receiver to be charged X amount" but they didn't bother knocking to ask for the money.