Does the post office return mail to the sender if the correct postage isn't on the letter?

I sent a letter regular priority mail and it hasn't gotten to its destination yet. I sent it Friday March 18th. I sent it out with one of those envelops the post office has with a 44 cent stamp already on it. The envelop only contains a single letter and the address I sent to is correct so what are the possibilities that it could of gotten lost or I didn't add enough postage and if that is the case will the post office return it to the sender?