Hi Eddy - Many thanks for your reply.
I had previously used Control Panel > Programs and Features to un-install my previous AV. I have now used an un-install tool from my previous AV vendor's site to remove any outstanding fragments.
Outlook 2010 uses the Windows Certificate Store rather than it's own certificate store. I checked that the avast! Mail/Shield Root certificate was in place and, as a precaution, exported the certificate from Avast then double-clicked on it to re-import it into Trusted Root Certification Authorities > Certificates.
I re-enabled Scan outbound email (SMTP) in Settings > Active Protection > Mail Shield and sent a test email to a webmail account. This worked perfectly so many thanks for this.
During all this I received a new email but, once again, could not view it in Outlook's Preview pane nor open it.
I also tried to do a Send/Receive All Folders in Outlook but, once again, the folder synchronising folders failed.
I disabled Mail Shield temporarily and I was both able to view my new email in Outlook's Preview pane and synchronise folders without any problems or errors.
I would be grateful for any more advice or suggestions.