Are trying to use a class that belongs to another model and you face this error : “The class or interface …. does not exist”
Don’t worry this happens because you forgot to reference the model in your custom model.
To solve the issue follow these steps :
Go to Dynamics 365 > Model management > Update model parameters
Choose your own custom model and press Next
Choose the model you want to reference (in my case it’s Docentric AX Emails) and click Next
Now rebuild your model