I think you can store emails but process them only in an anonymizing publicly auditable proxy, ensuring that downstream business services do not have plaintext access whilst still being able to send outbound emails whenever you want. I wrote about it recently: https://futurice.com/blog/trustworthy-services-from-cloud-pr...

The key is to grant cloudfunctions.functions.sourceCodeGet (Or AWS/Azure equivalent) on the edge so anybody can verify that your proxy is above board. End users just have to trust the Cloud Providers access controls, not the service providers word on implementation.