We've spend time debugging this and talking to our service provider's[1] support, but haven't been able to figure out the root cause.

If there are any experts reading this, we'd be grateful if you could let us know what we're doing wrong here.

We're in the middle of switching over to SES because they are landing flawlessly into Gmail inboxes, but it would still be good to understand what we could have done better.

[1]: https://www.zoho.com/zeptomail

The problem is your service provider, Zoho. Zoho is used by a massive amount of spammers. As a result, many email blocking and verification services will flag any MX records that lead back to Zoho.

Thanks for the information. This is sad if true. In my limited experience Zoho been an honest business to deal with.

Hello Vishnu, Truly sorry that you are facing this trouble and we are here to help. Please do share your Zoho account details and an elaborate description of the issue you are facing here - https://www.zoho.com/zeptomail/contact.html?src=pd-menu. Or you may also write an email to [email protected]. Rest assured, we will get to the bottom of it and help in resolving the same. Looking forward to your email. - Regards, Zoho Cares.