The base problem was recepients receiving multiple copies of an email. Since
one of the ten recipients was generating a "delivery failure notice" I
assumed this was generating a repeat sending of the email to all ten
recipients.
Deleating from the outbox did stop the "delivery failure" issue.
Other, related posts to multiple email deliveries suggest fixes:
Outgoing email scanning off... mine is off (using AVGFree)
Increasing server timeout setting... when going through this routine "email
account" then "more options" there is no "advanced tab" available in my
window.
What type of account? What version of Outlook? There is no "E-mail Account",
"More Options" path in any Outlook. For Outlook 2002/2003, it's Tools>E-mail
Accounts>Next, select the account, click Change, then "More Settings", then
the Advanced tab. In Outlook 2007, it's Tools>Account Settings, the E-mail
Account tab, select the account, click Change, then "More Settings", then the
Advanced tab.
Is there another way to modify the server timeout setting?
Perhaps you didn't click the correct things. Also examine your Send/Receive
interval (press Alt-Ctrl-S while in Outlook). Make sure the "Schedule an
automatic send/receive every" value is no less than about ten minutes.