The message staring out at me from my Gmail inbox said Id received an update on my previous conversation with a sender named vaishali The (3) next to the senders name suggested that I had responded to this person before, although I didnt recognize the name. As it happens, Gmail assigned the (3) to the message suggesting a threaded conversation because the sender had sent the same missive three times in a row.”]
Source: https://krebsonsecurity.com/2010/01/clever-gmail-spam-technique/