system
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Dear Technical 
You are quite right in that only unread messages will be scanned on highlight – I hadn’t noticed that and the oprtion was ticked. There is no option to “scan on open only” however, which is what I want. I subscribe to a number of mailing lists and in some there are hundreds (yes, even tousands) of unread messages (I won’t even blush) and they are scanned every time they are highlighted. If I by mistake mark too many of them (or if I want to move them to another Outlook folder), I will have to use Task Manager to abort Outlook.
Are there other ways to prevent this from happening? It seems little point in Avast rescanning messages that are not being opened over and over.
all the best,
– Inge