The Asset Type that should be used for storing email addresses of the process owners is Text. Text assets are used to store only string values, such as names, addresses, URLs, etc. Text assets can be easily retrieved and updated using the Get Asset and Set Asset activities in UiPath Studio. Text assets are suitable for storing email addresses, as they can contain alphanumeric characters, symbols, and special characters. Text assets can also be used in various activities that require string input or output, such as Send Email, Write Line, Assign, etc.
References: About Assets and Assets in UiPath Orchestrator and their usage in project from UiPath documentation and RPA Learners.