Default checked box when replying to PM

[Caravan]

I had just typed a long PM in reply to someone's msg, then after sending I thought there was an error as it did not showed up in the Sent Items.

Later I realized that there is this checked box 'Add a copy of... to my sent items folder', which somehow by default is not enabled when replying to PMs.

I think it is better that it be checked by default. I thought that is the default actions of many forums and email programs. Some users, like me, will forget or miss out on that box, and after sending out, thought the PM was not sent successfully (I retyped the long message again as I thought it wasn't sent out).

Thanks.