This symbol, Unicode decimal 12306, is the official logo of Japan Post, and appears on post offices, mailboxes, stores that sell stamps and postcards, and in writing to denote zip codes (I once lived in 〒565-0083). It is called 郵便記号 yûbin kigo or 郵便マーク yûbin mâku in Japanese, and is entered in most input method editors by typing "yuubin." In case you don't have it on your computer, it looks like this:

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Two theories circulate over the origin of this symbol. One states that it comes from the katakana syllable テ te, the first character in the postal ministry's original 1800's name, Teishinsho. The other states that the logo was originally a capital "T," before the agency discovered that "T" was an international symbol for insufficient postage, and modified the mark by adding an extra line.