～通
～つう · ～tsuu · JLPT N2 · noun
letters
✉️ 通 = pass through: each letter passes through the post to you (tsuu)
通 means to pass through or communicate, as in 交通 (traffic) and 通信 (correspondence). A letter is one thing that travels through the mail, so you count them with 通: 一通 is one letter, 二通 two. Emails count the same way.
通 — 辵 (辶) walk + 甬 yǒng (phonetic)
通 is a semantic-phonetic compound. The 辶 (chuò) component means 'walk' or 'movement', giving the meaning of going through or passing. 甬 (yǒng), originally depicting a hanging bell or a path, supplies the sound. Together they convey 'to pass through, communicate, connect'.