Yes, it is becoming increasingly difficult to persuade gmail etc. that
you are not a spammer if you run your own mail server. If you have any
interesting headers suggesting exactly what they disliked about my message,
could you please forward them off-list? Thanks.
It is for this reason (and few others) I am off gmail and other free email accounts.
I have tried gmail / outlook / yahoo / aol and all of them mark many mails to this list
as spam.
I like fastmail a lot and $3 per month is practically free.