Unless you have substantial volume to gmail (hundreds of mails a day), I can virtually guarantee gmail is spam boxing your mails, try mailing an account you have never sent mail before for instance.
They don't refuse delivery, but that is entirely meaningless when nobody ever checks the Spam box anyways (which is also hidden by default)
Would you mind sharing where you host your email server?
I have a pet theory that big email platforms care deeply about the IP "neighborhood" the sending email server lives in. So a small email server set up perfectly in Digital Ocean IP space would struggle, while an email server set up OK in (for example) CenturyLink Enterprise IP space would get through.
They don't refuse delivery, but that is entirely meaningless when nobody ever checks the Spam box anyways (which is also hidden by default)