I think it is Office Outlook 2003
When this message comes, I have to resend -often 2-3 times - it may go through or in some cases, I can't send it. Problem has been going on for several months.
Turn off Norton's e-mail scanning. Yes, really. You don't need it and it tends to cause these kinds of problems. Note: you may have to actually uninstall Norton and reinstall it minus the e-mail module to completely turn off the e-mail scanning.
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Sorry this didn't help.