I don’t use Outlook, but would it not be better to set the Inbound Mail tab, action to ‘move it to the following folder’ and create a suspect/infected folder to contain those. I don’t know if that would

Looking at the Silent mode it sends it to the chest, which is almost the same as deleting it as I doubt avast can neatly insert (if you wanted to restore it) the email into the .pst file so it is effectively lost to you. Interestingly there isn’t anything about silent mode in the avast help file, resident protection, outlook/exchange, advaced page info.