Comment by gerdesj

Have you got this lot sorted out:

  MX->A->PTR->A->MX
  SPF
  DKIM
  DMARC
  mta-sts - DNS and webpage

Also your IPs must be squeaky bum clean, ideally for several years. DNSSEC might help too. In the UK getting as far as DKIM is usually enough (plus clean IPs, even FTTC connections will work if static).