We configured “Verification Email” in Email Templates and set “Redirect To” field as “https://OUR_WEBSITE/login” and this works fine.
However, if a user uses a smartphone we would like that when you confirm the email our mobile application is opened instead of the browser as for the desktop.
We configured the device settings and we are able to “redirect to” our app but then the link to the website does not work.
Is there a way to open the browser if somebody does not have the app installed and to open the app if it is installed?
Unless an application is installed on a device that knows how to handle a deep link (which it sounds like you are using), then you’ll probably receive an error message rather than being able to default to a device’s browser. You would need to implement App Links (Android) or Universal Links (iOS), which would allow you to use a HTTPS url as your redirect url, which can be handled regardless if your app is installed or not.
Additionally, some email clients will prevent non HTTP/HTTPS urls from being clickable (for security).
It’s worth noting that App Links and Universal Links also try to improve security by working in conjunction with your web server to prevent malicious apps from handling your custom schemes/Urls.
This is a big topic, but hopefully these links will get you started: