I have a site that was upgraded from D6 to D7 and is now hosted on Pantheon. I'm using the SMTP module and SendGrid for deliverability. The "require email verification when a visitor creates an account" setting is checked. When someone registers as a new user, the message saying that an email has been sent to their account is displayed on screen. However, neither watchdog nor Maillog shows any evidence that an email was sent, and no email is received.
Other emails, including password reset, Webform emails, Comment Notify, etc. all are sent and received as expected.
What could be causing the new user email not to be triggered?