If the Uri is “mailto”, the Uri format must be in the following format.
mailto:firstname.lastname@example.org?subject=Application Feedback&body=Hi there, I really liked your app.
I encountered following error
System.UriFormatException: Invalid URI: The hostname could not be parsed.
It was difficult to find the root cause. Finally I found that I used “&subject” (ampersand) for the first parameter instead of “?subject” (question mark). Once I used “?subject”, everything worked as expected.