Note: Email insights is not supported in composer for email.
In some scenarios, if 50% of emails are opened by iPhone and then 20% by Gmail, Email Insights Reporting can differentiate the devices as follows: the Opens made through the default Mail app on a device show up as that device (i.e. iPhone). Opens made via some third-party apps, or with the mobile browser on a smartphone can be reported as a subset of webmail opens (i.e. "Gmail using iPhone'), or as the default or native mail client.