Shabbos 19 – שבת יט

Bais Hillel allow sending a letter if there is sufficient time to reach the first house in the city, for we may rely on the possibility that the Postmaster is present there.  Why (when there is no Postmaster) do they prohibit sending the letter, on the concern that he may have to spend time looking for the recipient; why can’t we rely on the possibility that he will encounter the recipient immediately upon his arrival in the city?