The unread icon for PMs is an open letter. Unread ones are sealed. Not sure if this is right, or not.
My offering if you're considering changing them anyway: Pesterchum indication icons (fattened up a bit to make the most of the 16x16 offered by the original icons)
And then because it didn't contrast as well against the background, with slightly darker colours:
If there's any other icons other than those ones, I'd be happy to try and throw something together if you like the look of these
Edit: Oh, also while I'm here being annoying, please consider making the size of reply titles less obtrusive.