I too like whocares am surprised that it didn't delete the local inbox.
Did avast successfully remove the virus, leaving the local inbox folder intact?
If so TB may be storing the emails individually in this local folders inbox option, rather than multiple emails inside one file (inbox.

). avast! obviously can't extract a single email from within a file of multiple emails, but can remove a single infected email if it is in a local folder (a proper folder on the HDD not a file pretending to be a folder).
Do you know the location of the local inbox you can check if emails are stored individually there.