Just wondering if this major bug was fixed in iOS7?
It was little things like this that have made me ditch the iPhone, and move away from the bundled Mountain Lion apps. (Sent emails not getting saved to the Sent folder every so often was another deal breaker for me). Thunderbird works so much better, while not quite and polished aethetically).
Android takes a bit of getting used to but once configured to your personal taste (that's the beauty of it) it works excellently.
Still think they make awesome hardware though!