Mailboxes Etc. — Eruvin 77a
תלמוד בבלי מסכת עירובין דף עז/א
דכי אתא רב דימי אמר רבי יוחנן מקום שאין בו ארבעה על ארבעה מותר לבני רשות הרבים ולבני רשות היחיד לכתף עליו ובלבד שלא יחליפו
A space that is smaller in area than four by four tefachim (but more than three tefachim above the ground) is a makom patur. This is also the case when a hole in a wall abuts a reshus ha'rabbim. It is not included in the reshus ha'rabbim, and if it is less that four by four it is a makom patur.
It is permissible to remove or bring objects from anything defined as a makom patur to a reshus ha'yachid or a reshus ha'rabbim or vice versa. One is not allowed, however, to remove an object from a reshus ha'yachid, bring it into a makom patur, and then transfer it through the makom patur to a reshus ha'rabbim or vice versa. If one stands in a reshus ha'yachid and removes, brings, extends or throws an object into a reshus ha'rabbim via a makom patur or vice versa, one transgresses a Torah prohibition. Similarly, one may not position oneself in a makom patur in order to take objects from an individual in a reshus ha'rabbim and give them to a third party in a reshus ha'yachid or vice versa. Even if the makom patur is located between two domains between which it is only rabbinically forbidden to transfer objects, such as two yards that have not joined in a common eruv, one may not stand in that makom patur and transport objects from yard to yard (if those objects originated or will terminate in one of the houses fronting on the respective yards).
On the basis of our Gemara, the Semag (Lavin§65) adds that it is also forbidden (by Rabbinic decree) for a person in the reshus ha'rabbim (or, by rabbinic decree, a carmelis) to place an object in a makom patur for another person on the other side to then take it into the reshus ha'yachid. The Sages forbade this case lest one of the people would come to perform all the steps himself, thus resulting in a Torah prohibition (see Shulchan Aruch,Orach Chaim 372:6).
It would thus seem forbidden to take in mail that was left by the mailman in a mailbox that is a makom patur between a reshus ha'rabbim or carmelis and a makom patur. However, Teshuvos B'Tzel HaChochmah () suggests that the Semag's prohibition only applies in a case involving two Jews. If the mailman is not Jewish, the case is not subject to the Sages' decree, and it would be permitted for the Jew to bring his mail in from the mailbox.