Recently, having not made any code changes, emails I've sent using the (SendEmailFromTemplate) API have started failing with the message. "Messages missing a valid messageId header are not 550 5.7.1 accepted".
A quick search with Google suggests this may be to do with a change to gmail in the last couple of months (possibly as a spam prevention measure, although I'm not sending spam!).
Is there any way I can define a valid messageId header through Playfab?