I got around the problem by moving from SendGrid to ElasticEmail.
I’m also getting the same problem when using Mailpoet with Sendgrid. When sending a test email from within Mailpoet settings, the message says it was successful, but the email never arrives. When you check within the Sendgrid console you find that the emails bounced with the “550 Messages should have one or no Content-Type headers, not 2” message.
