Mounted carriers must dismount and deliver the mail at the doors of residences, except in cases where the patrons on their routes consent to respond to their call and receive the mail at the sidewalk. Carriers are not required to deliver mail at residences where vicious dogs are permitted to run at large. Persons keeping such dogs must call at the post office for their mail. Carriers must collect and promptly return to the postmaster all postage due on mail intrusted to them for delivery, as indicated by the postage-due stamps attached. Such mail matter must not be delivered until the postage due shall have been paid. When carriers making collections from letter boxes find that it will be impossible on any one trip to carry to the post office the contents of all the boxes on their routes, preference must be given to mail matter of the first class. Newspapers and packages placed on the tops of letter boxes should be collected when it can be done without overloading the mail sacks and preventing the prompt collection of mail matter properly deposited in the boxes. Carriers must, while on their routes, receive letters with postage stamps affixed, handed them for mailing, but they should not delay the deliveries by waiting for such letters. Money to pay postage on letters handed them for mailing must not be accepted, except as provided for in the use of the stamp-selling envelope in connection with house-to-house delivery and collection boxes. Carriers should also receive other small articles of mailable matter with postage properly prepaid, but they should refuse to receive packages that are cumbersome on account of size, shape or weight, especially when the carrying of such packages would interfere with the prompt delivery of mail and the collections from letter boxes. Carriers must receive and register all letters and packages of first-class matter that are not cumbersome on account of size, shape or weight, and properly offered them for registration, and must give the regulation receipt therefor. Carriers must encourage the registration of valuable first-class matter by patrons on their routes. Postmasters may permit carriers to sell postage stamps or stamped envelopes in limited quantities; but their deliveries or collections must not be delayed in making change. Carriers must not return, under any circumstances, to any person, any letter or letters, said to have been deposited in a letter box, or which have come into the custody of the carrier in a regular way. An applicant for the return of such mail should be directed to the postmaster.