Customers not receiving emails

Customers not receiving emails

So, a little backstory, we have been using Zoho Forms for the past eight years. And when we initially started, we would have email notifications be sent from inside Zoho Forms after a submission. We recently started using Zoho CRM as we wanted a better system to keep track of our records. And in adding Zoho CRM, we decided to have emails be sent from within Zoho CRM via a workflow so they would be attached to the customer record. I've noticed that when sending emails from Zoho CRM, they seem to "send" and be marked as "delivered," but the customer states they never got them. We have gone through DKIM and SPF and tested using an email relay, but nothing really seems to be reliable. And the settings panel shows the domain is set up correctly. I've also noticed, sometimes emails are marked "Open" within 1 minute of being sent, and then the customer said they never got it? This isn't to say that all of our emails are failing, but a larger portion of them seem not to be going through. Any thoughts? When we sent them through Zoho Forms, we rarely had any reports of not receiving emails.