Is it a security risk to view an .eml file in a viewer?

This is not similar to Are .eml attachments a security risk? as the .eml file doesn’t any attachment.

My supplier has some email issues he is sending so I sent him the .eml files to check it. Even I am doing him a favor he says that he is not going to view the .eml files with a viewer due to security risks and I have to understand it.

My questions:

Is there any know security issue viewing an .eml file by a viewer, e.q. not in Outlook?
Or, what can theoretically happen when the .eml file is viewed by any external viewer?

The .eml files I would send are generated by my Exchange based on emails he sent me and do not contain any attachment.