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You don’t say what version of avast you are using. These automated options are only available in the Pro version. This is a limitation of the Home (free) version that it has interactive input requirement, there have to be differences in the Home/Pro version and this is one of them, the programmers have to eat ;D

You only have a limited option in the Home (free) version, to send the infected file to the virus Chest (silent mode, with general answer no, see below). Effectively an infected email sent to the chest is history as there is ‘no’ way to restore it as avast can’t insert it into a folder.

My own feeling on this is you should use the default interactive action. This way you know exactly what is going on with your system. If you are getting so many warnings, that you want to automate this process, I believe you should review your security practice - filter emails at source, delete from server rather than download them, review the sites they visit, etc.

See the avast help file, Resident Protection: Internet Maild Provider Settings - “Advanced” Page.
Click on Internet Mail and then on Customize.
Go to Advanced tab and select Silent Mode and the General answer No.

Leave the file in the chest for a few weeks (it can do no harm from there) to ensure no adverse effect from being moved to the chest (shouldn’t be as it is an email). Then delete it from the chest, although emails could theoretically they wouldn’t have an adverse effect and since you can’t restore it you could just periodically delete them.