Sending email to the chest as alanrf would say, is a waste of time and you might as well delete as you are wanting to do anyway. You would have to accept that there may be occasions when you are consigning a legitimate email to the bit bucket. You should also note that avast isn't an anti-spam tool so won't detect spam unless it is also carrying a payload that is either infected or suspicious, so it won't have any impact on what gets past your spam filter.
The reason why it is a waste sending emails to the chest is that even if you decide to restore one, it is unlikely that avast will be able to insert it into the email database file where it would have been. This may possibly corrupt the database file with the possible loss of any emails already in that file.
This however isn't the case for regular files on your HDD which should be sent to the chest and investigate.
