I've just upgraded from 13.04 to 13.10 and everything seems to be running fine. I have a couple new icons in my systray, though, and one of them -- the envelope icon -- is a mystery to me:

It looks like a chat notifier (clicking it gives "Available", "Away", "Busy", etc., indicators) but I don't use Pidgin (never have), or it could be an email indicator but I don't use Thunderbird (never have). There's nothing else I can see (left- or right-clicking) that identifies it.

I'd like to get rid of it, but "Startup applications" only lists the ones I have optionally added, and this is not among them.

Any help appreciated!