I won't say that e-mail viruses are impossible. But they are highly unlikely. They do not exist.
Viruses are executable computer programs.
E-mail messages are simply data. In order for computer viruses to do their mischief, they must be activated somehow. Reading an e-mail message will never activate a virus. Attachments, on the other hand, are a different story. OR data that people can send with a normal e-mail message.