I sent in a support request and received this message:
In order to receive the 100 emails per day, you'll need to choose the
Email API trial or the Marketing Campaigns trial. You can do so by
navigating to Settings > Account Details > Your Products > Start Trial to Send More.
If you start the trial you will receive 40,000 emails a month, but
From Don’t Send Email From Domains You Don’t Control, by SendGrid Team, dated January 3, 2017
If you’re sending from gmail.com, outlook.com, or some other third party domain that neither you nor the customer on whose behalf you’d like to send control, I have bad news. You’re going to have to find a new way. Starting very soon, those messages are never ...