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.