i can confirm that only with disabled service this works .... if i just disable incoming the behavior remains the same. point is that scripts in the website cannot use sendmail or something similar now.
this is a very serious problem and I believe it will become more of a problem as people little by little migrate mail on major mail providers.
one domain cannot send to another hosted on the same server if the second domain is hosted on google meaning mx records point somewhere else.
this means...
how is it possible my domain dkim is marked as fail in google gmail but on several dkim resolvers as EasyDMARC | DMARC Journey Made Simple or toolbox appear valid ?
i deleted the dns record then uncheck the Use DKIM spam protection system to sign outgoing email messages .... --> save -->...