My legitimate emails being tagged as junk or spam

I have 3 Apple IDs - '.mac' '.me' and '.icloud', since I have been with Apple a long time and started with dot Mac when we paid for the service. Recently some of my Mac emails to friends have suddenly been going into their junk folder, if they even reach their computer at all, and one friend checked the full headers of my emails to him, and found my email was being marked by something as junk. Either 'spf' or 'dkim' or 'dmarc' was getting a "fail" with some being put into quarantine. How do I get this corrected, or who do I contact to get it sorted? Is this something to do with the Apple server or something Apple are doing? I do use Thunderbird as my mail program, but I have done so for many years and that has never caused a problem before. Thanks for any help.