After switching to Salesforce Lightning, you receive the following error when trying to open a copy of a Marketo email: "You can't preview or edit Classic templates on the Email Templates home page. Go to Classic Email Templates in Setup."
Currently the email templates are saved under the "Classic Email Templates" and are not compatible with Salesforce Lighting. If you are trying to view the emails in Salesforce that are sent by Marketo, you will need to view the emails in Classic Mode.
Who This Solution Applies To
Customers using Salesforce Lightning