I happened to setup Zimbra as our new Client , set up dmarc, dkim, spf, but even at that the webmail by default sends emails directly to spam in Hotmail / Gmail, without even thinking twice.
What could be the cause? VPS server is hosted on Digital Ocean, and Cent OS 8 is the Linux server used.
A lot of scenarios may cause the mails to be forwarded to the Spam box:
I suggest that you send a test email with random content from your VPS to the email address you'll find on mail-tester.com then click the button to verify your score.
Any score below 8.5 would require that you take action; Required actions will be listed in the report generated by mailtester.