The following code will get your HTML out of your app and into Apple's Mail app. The documentation doesn't give you a great deal of help on this, so in part it's a matter of looking at what Apple's apps park on the pasteboard and then reverse engineering that. This solution draws on an earlier stackoverflow post - follow up the links there for more background.
NSLog(@"Place HTML on the pasteboard");
UIPasteboard* pasteboard = [UIPasteboard generalPasteboard];
NSString *htmlType = @"Apple Web Archive pasteboard type";
// example html string
NSString* htmlString = @"<p style="color:gray"> <a href=@"http://itunes.apple.com/gb/app/paragraft/id412998778?mt=8">Paragraft</a><br><em>Less than a word processor, more than plain text</em>";
NSMutableDictionary *resourceDictionary = [NSMutableDictionary dictionary];
[resourceDictionary setObject:[htmlString dataUsingEncoding:NSUTF8StringEncoding] forKey:@"WebResourceData"];
[resourceDictionary setObject:@"" forKey:@"WebResourceFrameName"];
[resourceDictionary setObject:@"text/html" forKey:@"WebResourceMIMEType"];
[resourceDictionary setObject:@"UTF-8" forKey:@"WebResourceTextEncodingName"];
[resourceDictionary setObject:@"about:blank" forKey:@"WebResourceURL"];
NSDictionary *containerDictionary = [NSDictionary dictionaryWithObjectsAndKeys:resourceDictionary, @"WebMainResource", nil];
NSDictionary *htmlItem = [NSDictionary dictionaryWithObjectsAndKeys:containerDictionary,htmlType,nil];
[pasteboard setItems: [NSArray arrayWithObjects: htmlItem, nil]];
// This approach draws on the blog post and comments at:
// http://mcmurrym.wordpress.com/2010/08/13/pasting-simplehtml-into-the-mail-app-ios/
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