massive spam problems with Zoho CRM emails sent via ZOHO Mail

massive spam problems with Zoho CRM emails sent via ZOHO Mail

As a company, we do NOT send ANY marketing emails. All emails are solely an answer to leads.
Nevertheless, we face massive MASSIVE problems with customers in Germany, Austria, Switzerland and the UK because emails are 
1. not even reaching SPAM folders (greylisting)
2. directly marked as SPAM 

Situation:  both SPF and DKIM are 100% correct.

Who else is suffering from those problems and how did you fix them?
Is leaving Zoho Mail the only possibility?

Thanks