Hi Stefano – I am receiving the same problem.
I am also using WP SMTP Pro version, which enables Microsoft Office 365 Graph API (which as a service is fine for sending newsletters)
Can we please raise this as a bug for investigation? I’ll do my best to add a filter as suggested above, but that’s a workaround and I’m not sure which headers I should be removing….
Happy to provide a test environment if you’d like to see for yourself?
