When are thank you eMails good manners and when are they spam?

For those of us who got manners it comes natural to say please and thank you. So every time I get a question answered in eMail, I have that reflex to hit the reply or even reply all button to say Thank you. I'm starting to wonder if that is still always appropriate. In IBM Connections we have a widget for " Giving thanks" that separates the process from the daily stream of eMail. Why do I think it might not be appropriate all the time: I just adds another item to a long list of items that want one's attention. Of course if the answer was a big help and you have to share how it helped a reply is indicated. Also you need to gauge the sender of an answer: do they fancy or expect a simple thank you for each message? I like the little thank you messages arriving in the blog comments or tweets and the occasionally larger thank you messages dropping in at quarter or year end, but I don't care too much about a thx in eMail for something obvious. And I definitely don't care for the thx someone else gives in a reply-to-all. After all I believe we all have manners by default . But I still wonder where and when to draw the line for that little thx in a reply.