This is from functionality added in SAK-9109. There's no way to set the mailtool.group.aware.role property to nothing because the defaults are hardcoded. If you set mailtool.group.aware.role to an empty string or a bogus role the default will be used. So basically the mailtool is forcing you to have a role configured that is group aware.

public String getGroupAwareRoleDefault() {
if (getSiteType().equals("course"))
return "Student";
if (getSiteType().equals("project"))
return "access";
return "";
}