I was really surprised when I read the e-mail because I haven't heard from you since I visited you last year.

since 在这里是连词，表示"从...时候起"，用来引出时间状语从句。它强调从过去某个时间点开始一直持续到现在的情况。 heard from 是动词短语，hear 本意是"听到"，from 表示"来自"，合起来表示"收到某人的消息"或"得到某人的音讯"。