So, I am trying to add a go-to-action button/label for emails for gmail customers in their inbox (see https://developers.google.com/gmail/actions/reference/go-to-action).
The email is sent via phpMailer.
I have successfully tried the example implementation for the view action within scripts.google.com. Unfortunately the example code won´t work for any email I am trying to send via phpMailer.
So far, I have checked the email markup successfully at https://www.google.com/webmasters/markup-tester/ without any errors.
After viewing the original sent email files I only recognized that the email is sent as text/plain and as text/html. Does anyone have the same issues? Any suggestions what might be wrong? Thanks in advance!