I believe the 1-1 personal emails were still sent through a mail service. In my experience (from being the sender for similar emails) those still get caught in promotions or spam a lot of the time.
A solution for this would be to send "normal" emails (not bulk, no images) from a "normal" email address (like Gmail). I'll definitely consider this for the 2018 survey.
For reference, the question we used for people to self-identify as an EA (and be included in the analysis) was:
Could you, however loosely, be described as "an Effective Altruist"? Please answer “Yes” if you are broadly in agreement with the core ideas associated with Effective Altruism, even if you don’t like applying the term to yourself (e.g. because it sounds presumptuous or you are an “aspiring Effective Altruist”).