Hello,Google tends to have quite strict email security, therefore it is no surprise it is marking some of the emails as spam.
Its not only dkim and spf they look at, but they analyse the content of each email. If you are running your own mail servers, it is very hard to not be spammed because google and other large companies all work in collaboration with one another to whitelist each other, but they spam smaller email services.
Another example of how protocol(s) designed to be decentralised, has been centralised by major companies through making it increasingly difficult to host your own services.
I do not know why google is spamming emails, so I can't help there, but most likely it is just due to an email server being small and thus google instantly spamming it.
Polarian
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