This question was answered by @ahrotahntee:

There’s a checklist for ensuring you don’t end up as spam:

• Check MX Blacklists, see if your box is present on them
• Make sure your PTR is valid
• Make sure your mailserver isn’t identifying itself as ‘localhost’
• Make sure that your SPF record is set and is allowing your box to send mail
• Make sure that you are not actually spamming

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Hello, all

You can make sure that your mailing list is fully opted-in and that you’re using SMTP authentication.

Also to test the mail score of your emails you can use a website like:

https://www.mail-tester.com/

and then send a valid test - an exact email with a valid - subject, message body, full signatures and attachments.

Regards,
Alex