Windows XP Pro SP2

"The message could not be sent. An error has occurred."

Oh. Okay. Don't you just love these descriptive error messages!

I receive that error message when I press the Send button for an Outlook
Express 6 signed message. I have spent 19 months trying to solve this and
when SP3 comes out I will try a fresh re-install, but I still need to know
why.

I have Certificates from Comodo and Thawte.
I have re-installed the certificates many times.
I have tried 4 different E-Mail addresses.

Please help, thank you.
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Help me help others, ju.c.