1. Make sure your IP have reverse-dns (ask to your provider) if not exists
2. Make sure already enable 'domain keys' (set via 'admin > server mail settings'
3. Make sure your domain already set their SPF and DMARC (set via 'mail auth' for every domain)
4. Decrease 'Score At Which Mail Is Judged Spam' from 5 to 7 until 9 (set via 'Spam Status' for every domains)
5. Check your IP in mxtoolbox.com