Outlook - Email sending not complete....

Laptop with XP and Outlook 2007 shows email sent to correct email address.
Email not being received by recipient.
Email address has been entered manually as well as from contact list.  Both
addresses are the correct address for recipient.
All other emails being sent from laptop are going through just fine.
No rule found to reroute or change on sending machine.
No rule on recipient machine to divert or delete from this sender.
No servers involved.
Email address and machines have been working perfectly for many years.
Where and what else can we check?

If your message gets sent to the outgoing server (Outlook shows it in the Sent
Items folder), you do not get a non-delivery report after some period of time
(should be at most three days), and your receipient never gets the mesage,
then there is a routing issue somewhere between your mail server and that of
your recipient or you have interfering software on your PC.

The latter is much easier to check.  The first thing is to make sure you do
not have an antivirus scanner cauing the problem.  Uninstall your AV program
and reinstall it without any mail scanning feature.  Doing so does not
decrease your safety.  After doing this, try sending another message to see if
it gets through.  Do not conclude it did not work until you have waited, say, 24
hours.  E-mail is not and never has been instantaneous.

If the message still fails to show up, then you will have to work with your mail
service provider to see if they can find the downstream service interruption.

Okay I understand, however this is email going out and coming into the same
mail server.  It is internal in an office without a server.  Other parties in
office with email address through same mail provider are receiving mail just
fine.
Can Norton be the culprit for just one address all of a sudden?

Narrowing it down....  this all began with an email into computer #1 being
forwarded to computer #2

Sending a plain new message to computer #2 from computer #1 and including a
computer #3 all seems normal

Re forwarding the 1st email to both computer #2 and #3 results in the email
appearing in the sent items of Computer #1 but no receipt by either #2 or #3

What could be in this email that will not allow it to be forwarded.

You say "internal in an office without a server".  That's simply impossible.
Outlook must get its incoming messages from a server and send its outgoing
messages to a server.  It cannot communicate directly with another instance of
Outlook.  What type of account are you using?  POP?  IMAP?  Exchange?
--
Brian Tillman [MVP-Outlook]