Thanks David. Install/set/uninstall of mail shield worked fine.
The entries in emailshield.ini are:
InsertNoteCleanMsgIn=0
InsertNoteCleanMsgOut=0
InsertNoteInfectedMsgIn=0
in the:
[EmailScanner]
section.
Hopefully the Avast people will add this to the web shield configuration, as it is an annoyance to have to do installs or edit the .ini file.