Swift:

if let url = URL(string: "mailto://[email protected]") {
    if UIApplication.shared.canOpenURL(url) {
        UIApplication.shared.openURL(url)
    } else {
        print("Cannot open URL")
    }
}

Objective-C:

NSURL *url = [NSURL URLWithString:@"mailto://[email protected]"];
if ([[UIApplication sharedApplication] canOpenURL:url]) {
    [[UIApplication sharedApplication] openURL:url];
} else {
    NSLog(@"Cannot open URL");
}