Personally, I hate this feature and the fact that you cannot turn it off – there is a checkbox in the preferences but it’s meaning is whether to delay the marking – if you uncheck it, then the message is marked immediately. I typically have to check the box and put in a large delay but even that seems broken in TB 1.5 as it seems to choke on the large numbers.
If this annoys you too, please vote for the following bug: