It is always better to check the envelope or the letter properly in terms of its weight dimensions to meet the qualification criteria of USPS while sending the mails. These help ensure that the postage which is due for that particular size and the specific envelope or the letter is attached to the mail. When the required postage is missing or you put a stamp that is not apt for the mail that you are trying to send it can happen that the same will not go through. The mail will be returned back undelivered. If you do not mention a return address on the envelope, you will not able to get back the too. It can also happen that the post office will put a stamp that mentions ‘postage due’ and return it back or keep it aside. In certain situations, it can also happen that the postmaster will affix the required stamp and let you know about the due via your mailbox. While certain other towns do not entertain such requests or situations and straight away returns back the mail.