(a) A letter may be carried out of the mails when - (1) it is enclosed in an envelope; (2) the amount of postage which would have been charged on the letter if it had been sent by mail is paid by stamps, or postage meter stamps, on the envelope; (3) the envelope is properly addressed; (4) the envelope is so sealed that the letter cannot be taken from it without defacing the envelope; (5) any stamps on the envelope are canceled in ink by the sender; and (6) the date of the letter, of its transmission or receipt by the carrier is endorsed on the envelope in ink. (b) The Postal Service may suspend the operation of any part of this section upon any mail route where the public interest requires the suspension.