CORS is an error raised because you’re trying to use a domain or subdomain (DNS) you have no permission for. It’s meant to prevent hackers to use something or server they are not allowed to use (cross domain hack).
iOS is very strict about that, more than Android. Anyways, you’ll still have the issue as long as you’re trying to send an email from an unregistered / unauthorized domain in a real production app.
For the rest, you can disable CORS policy for testing, there is a fix for iOS somewhere in forums.