email go-to-spam problem

Job ID: 33645065

Budget: €250 – €750 EUR

My direct emails (not bulk) are regularly blocked from sending (in client) or go into spam at receiver even after several contacts.
My domain is NOT reported as spamming (as far as I can check).
All other standard test and tools show no solution. My provider is sure it's a fault on our side.
Related categories: PHP Linux Web Security DNS Email Handling