Send e-mails to ExpressionEngine Mailing Lists subscribers with Postmaster and SendGrid.
Move Sendgrideemailinglist.php into third_party/postmaster/services/
This Service uses the x-smtpapi field to send to multiple receipients. The 'to' field on line 95 isn't used, however I recommend you change it to a real e-mail address just in case SendGrid ever choose to stop ignoring it. Currently it's set to dummy@example.com.