DeltaHF
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Interesting idea, although I suspect it would be very difficult to implement and, in my opinion, beyond the scope of this add-on.Don't know if it is possible but would be great if you don't recieve mails when online. When in active conversation you still have to remove a lot of mails.
Same for watched threads would be nice But that is something different.
It would be ideal behavior from XenForo, though, because I've experienced exactly what you are talking about so many times. This add-on does, at least, help reduce this problem somewhat.