The power "To establish post-offices and post-roads" (Const. Art. I, § 8, ¶ 7) has been fully exercised by Congress in creating a post-office department of the federal government, under which the transportation of mail matter is generally and economically effected. Very few questions have arisen with reference to the exercise of this power, and there has never been any conflict in that respect between federal and state authority, the power and practice of the United States to control the whole subject being fully recognized. Congress may undoubtedly make railroads, steamship lines, and other methods of transporting the mails, post-roads; and thereby bring such means of conveyance under federal control. Indeed, Congress might no doubt go further than it has already gone in establishing and controlling lines for mail and telegraphic communication. As railroad, steamship, and telegraph lines are almost universally engaged in carrying on interstate and foreign commerce as well as in transporting the mails, Congress has a double power of regulation.

One important question has, however, arisen as to the extent to which Congress may authorize the exclusion from the mails of matter which is deemed injurious to the public. Under the power to establish post-offices and post-roads, it has been provided that obscene matter, advertisements of lotteries, and communications calculated to defraud or in their nature libellous, may be excluded, and this has been held to be within its power (Ex parte Jackson), the discretion to determine what matter is improper being left to the post-office department. A question may well arise as to the extent to which this discretion can be exercised. So long as the matter excluded is matter which a reasonable public policy declares to be improper, there can be no question; but if Congress should attempt to exclude from the mails matter which is essentially proper and not injurious, it might well be questioned whether the restriction would not be unlawful, perhaps under the general requirement that laws should be uniform in their operation, and that all persons are entitled to the equal protection of the laws. It is settled (see In re Rapier) as to lottery companies that even though chartered by the state in which they are operated, they may be excluded from United States postal facilities. (See § 216.)