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There is no golden advise. The more people move their emails to the big providers (Google.Microsoft), the issues with real SPAM is pushing theses provider to more automatic tools to identify SPAM.We followed all advises, we make clean invitations (only one link, no images), but still our invitations to outlook.com and to hotmail.com addresses go to spam-boxes.
So more and more legitimate emails are placed into SPAM. More is placed into quarantine and more often mailservers get completely blocked.
The internal SPAM and quarantine filters of Google,Microsoft and others are not really looking at these things.I have SPF + DKIM + DMARC … all OK. Seem better since i have some user asking to set me as ham.
There is an ongoing "war" to revoke the right to maintain your own mailserver. By default the scoring models of big provider will score a new mailserver and low volume mailserver as a threat.