considering both the organization's and the cloud provider's shared responsibility for security, SaaS (Software as a Service) would likely be the most suitable option for moving email services to the cloud.
In a SaaS model, the cloud provider takes on more of the responsibility for infrastructure and security, including protecting the underlying platform and application. The organization's responsibility primarily lies in securing user access and usage of the application, which aligns well with email services.
With SaaS, the organization can still configure and manage user access and policies, ensuring the security of sensitive data within the email service while benefiting from the cloud provider's security measures for the underlying infrastructure.