Check your PM options --
... to make sure you've requested that a copy be saved in your Sent Items folder.
If you didn't get an error message when you sent your PM, I'd assume that it got sent. 500 characters isn't a very long PM. I've sent some that were quite long and had no problems with them.
If you'd like someone to experiment with, I'm game. Feel free to send me a couple of PMs; nonsense text is just fine since all you're concerned about is whether the PMs are delivered and maybe how long they can be.