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.
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.
They use there own internal tools which are often based on metrics (volume, time, IP reputation) etc. You won't get any information about why the scoring happend that way. And often the provider don't know as well.
The selling proposition of Microsoft365 and Google is, that they will care about emailhandling. But they don't define rules per customer. They have basic, global rules and allow customers to deactivate them and create their own. Most customers won't do that. They won't look into the logs or quarantine tools. So more and more legitimate emails are placed into SPAM. More is placed into quarantine and more often mailservers get completely blocked. Which is always presented as you fault. Never the fault of Microsoft or other big providers. Which is a lie.
The lie is, that good mail management is scaling and that big providers can maintain the mailserver for millions of customers with their staff.
The next level now is that the SPAM is no longer coming from outside, but from within.
If you mail survey invitations to private / free mails, you can buy yourself into Whitelist mailprovider. E.g.
certified-senders.org/
