I run Vista Home Premium, and use Outlook 2003 for its advantages of
combining mail, contacts and calendar etc., with the ability to synchronise
with an iPAQ.

Recently, I discovered that a number of emails I had sent never arrived,
though they appeared in the ‘Sent Items’ folder.  

First, I tried a ‘System Restore to about 10 days earlier, but this never
completed and I was forced to return to the original date.

The only change that seems to have occurred around that time was a regular
update of Norton 360 or an overnight automatic Vista update and restart.

After reporting the problem to my ISP (Talktalk), a technical guy called me
and a spent an hour on the phone trying various tests.  By getting me to use
Talktalk Webmail to send a message to myself, he said it confirmed that the
server was operating properly.  Then by sending messages by Talktalk Webmail
to myself on Outlook and a Googlemail account I use, that messages get
through to Googlemail, Webmail and Outlook.  

However they found that if, in sending a message in Outlook, I click’ New’
then ‘To’ and select an address from the drop down menu, the message
vanishes, though it appears in the ‘Sent Items’ folder.  

If I click ‘New’ and enter the recipients address manually the message
arrives OK.  From this they deduced that Norton 360 was, in some way blocking
outgoing emails and suggested contacting Norton.

Norton said this was unlikely but suggested temporarily de-installing Norton
360 to see if the problem persisted.  It did persist, so for safety, I have
re-installed Norton.

With the advantages of Outlook it is annoying to have to enter addresses
manually and request a receipt or switch to another mailing programme with
which I am not familiar.  

Any ideas?