Where do you host? what class of IP you have? I had some issues with Gmail but adding rDNS IPv6 and enlisting into DMARC fixed it. I sent about 125k per week,

Where do you host? what class of IP you have? I had some issues with Gmail but adding rDNS IPv6 and enlisting into DMARC fixed it. I sent about 125k per week, 80% gmail, no serious issues (I would known cause users pay for email's content).