Interface Error with MS Outlook 2000

Win2K w Svc-Pk 2
MS Outlook 2000 Svc Pk 1

The Error (A Dialog Box Pops up.)
“The messaging interface has returned an unknown error. If the problem persists, restart Outlook.”

This error would normally happens when I type a few paragraphs of text. And the moment I click on the “Send”, the error appears.

It generally don’t happen when I type a short reply mail or when I send a short email. It really kills me, when I have to write long mails to colleagues.
Please help.