No Transport Provider Available using Outlook 2000 and Exchange 5.5

1/3 of our users are not able to send any email via Outlook 2000 and Exchange 5.5 but they can via Outlook Express.  They receive a message back stating that No Transport Provider is available.

I have setup an Outlook profile that just has a PST file and an internet mail sservice which also does not work.
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in outlook 2000 u must add the exchange transport for ur mapi clients inorder for them
to be able to send through exchange server.(i dont have outlook 2000 right now)but if
u right click on outlook\properties u will see the option.for troubled users check to see
if that option is added in their outlook profile.

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You receive a "No transport provider was available for delivery to this recipient" error message when you try to send an e-mail message in Outlook 2000
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